>

I use a corne (40% split ergo) as my daily driver, at home, as well as at work. I think the form factor depends on how you use your computer and how you are able/willing to adapt. My first mk was a 60% keyboard and that was pretty easy. You mainly lose F keys, arrows and number block, but do you really? Capslock is a useless key, so it becoms a layer key, if i push it my home row becomes arrow keys. I dont have to move my hand for arrows, how cool is that? Im not using many F keys but the number row become F keys on that layer. Fn makes a number block... 
On the corne you loose the numbers but gain thumb keys. This was a harder step (60 was easy). Now you start working with even more layers. There you really have to experiment, train, adapt. I really like my layout now and it works for me but I would advice to not go lower 60% if you are not up for a challenge. But yea, the lower the key count the lower amount of hand movement...
Why would anyone who's using a desktop computer spend hundreds of dollars on a keyboard that has less functionality than a cheap (realistically free) office keyboard?

This comment explains the reasoning behind why mechanical keyboards are better than the e-waste keyboards that Dell, HP, etc. include for free. I suggest reading through the buying guide found in the subreddit's resources because it as a lot of information.
TLDR - customizability, durability, aesthetics, and typing feel are the big reasons why custom mechanical keyboards have so much popularity. The market has been rapidly expanding for the past 2-3 years, so it's a better time than ever to get your own.
Smaller keyboards are sometimes cheaper, and thanks to layers they offer the same functionality of their larger counterparts. Software such as VIA and QMK are powerful tools to help with this. The big reason why I like small keyboards is they are visually appealing and because (most of the time) I have no use for the numpad and/or home cluster. >

I can't view the link that was sent because it says the comment was deleted.
I appreciate the response. I guess for the entry level this makes a ton of sense, lowering the cost to get into the hobby is great. >

The post was deleted, but the comment is still there. I used the share button on the comment but I guess it just loads the post lol 
Look for the super huge comment by EMCoupling, it has a few awards. >

Found it! I appreciate the info, thank you. >

Don’t know if this has been said but people who use smaller layouts are opting to just use layers (having to move their hands less further) rather than using a tkl and having to move there hands far to press f12 >

Ah, alright. I appreciate the info. >

A 60% is the alphanumeric cluster and what you probably spend 90% of the time using on a full sized keyboard, at least if you are not a number cruncher.
Pretty much every 60% will have a function layer that gives access to at least a TKL in functionality.  The F row will be the number row.  WASD or IJKL will be the arrows.  Backspace will often be Delete.  Other navigation keys will be mapped somewhere.  Most will have multimedia keys as well.  Those that have multiple function layers will let you even have a numberpad on a layer or macros.
At this point you have smaller keyboard that does not need your hands moving as much.  And you can move your mouse closer (assuming a right handed mouse user which most people are).  Full size keyboard impose bad ergonomics.  Either you position the keyboard properly with the B key centered on your nose, or you tilt the keyboard or position it to the left.  So you end you up with either strain on your shoulder or your wrists. >

I personally love 65% or 60% the most. I type and work the fastest on this size. Using function button is much faster and easier for me using layer than reaching out the usual place. I even feel awkward now whenever I need to use full size. That being said, on my previous work where I needed to do many data input, I used separate numpad that I put on my left side instead.
So unlike the other comment said, at this point, I need a smaller keyboard to be more productive. Aesthetic more like a bonus point for me. However, I'm still not used to smaller than that. >

Interesting response, thanks for the comment. >

Smaller boards take up less space, use less parts and, most importantly, you have to move your fingers less to reach everything.
Layers make it possible. They also work on fullsize boards, but it's basically impossible to get used to them when there are physical keys for every action. >

Thanks for the info! >

At least for me, the reason that I like slightly smaller keyboards is because it gives me more mouse space when I'm playing games and in general. 40% is too little for me, but there are some 60% with arrow keys layouts that work well for me because I don't find myself using other keys that often. Also, I find that small keyboards like the 60% hhkb I have are more portable than you might expect. Even when I use larger boards, I basically never use the extra keys they give me, so there's no reason not to just save on desk space. Of course, if you use all that functionality a lot and you have a lot of desk space, then there's no reason to downsize. 
To add to that, I think people who go through the effort of making their own keyboard tend to prefer things that they can't just get in a cheap office keyboard. I think the fact that you can't really get a cheap 40% keyboard is why you see so many custom models - after all, this entire hobby is very niche. Seeing how small a keyboard you can use is a hobby in itself for some people. I've also heard arguments that having things on multiple function layers on keys that are close to where you rest your hands for typing is more ergonomic since you don't have to move your hand as much to hit certain things. For people who do want more functionality, there are things like separate numpads and macro pads you could use and position anywhere you want. >

I'm genuinely surprised your desk is small enough that the 5-10cm shorter keyboard makes a difference. Do you mind posting a picture of your setup? I've never felt very space constrained by my keyboard before, so this is very interesting to me. >

Alright! Thanks for the post! I feel I understand the usecase quite a bit better now. >

I use a 60% because it's more comfortable to me than bigger layouts like tkl or full size. The numbers on top are good enough and I don't ever use the F keys on top of those number keys. With a 60%, if I wanted to use the F keys I just have to press FN + one of the numbers on top to get them. I have my keyboard tilted so it's more comfortable and I don't think there is enough room for a bigger keyboard like a tkl or full sized one. I also think bigger keyboards like tkl or full sized ones look weird when titled. >

Thank you for the info! ^_^ >

i went from 75% > 65% > 60% > 40%. on a keyboard >40%, my fingers have to stretch out more to reach the numrow but on my corne (40%), typing numbers are just one key above the home row using layers. 
all the symbols and function keys are also accessible through layers which works for me as a programmer and you get used to it over time. i also prefer a smaller board for travel as the corne fits perfectly on the wrist rest/touchpad area of my laptop. >

Is typing using layers faster or slower than typing with a larger keyboard with a dedicated numrow or numpad and or function keys? Thank you for the unique perspective. >

personally, i feel that i program faster with the symbols and function keys all kept within 1 row from the home row. the columnar staggered layout has also helped with touch typing and i rarely look down on the board to find the key i need. i don’t think i would be able to be comfortable with a non split 40% as i like having the function keys near my thumbs’ resting position.
a numpad would probably be faster if constant number inputs are required, but you can also have numbers positioned like it is on a numpad on a separate layer. >

Ah, this makes a ton of sense actually, thanks for the post. >

Minivans and wagons are way more practical than almost any vehicle on the road and yet people still buy coupes lifted trucks that they'll never put anything in the back because it could get dirty or scratched. >

So from your perspective, 40 and 60 percent keyboards are impractical and are largely a cultural and aesthetically based purchase instead of a functional one? >

I've never seen a keyboard like this before, this is super interesting. Thanks for the info! If I build a keyboard in the future, I might do something like this. What do you call this / how do I find a layout that supports this type of functionality?
Edit: This example really did it for me I think. I'm pretty convinced at this point. >

This is a case of "exactly how far down the rabbit hole do you want to fall?"
Assuming you want to avoid the headfirst dive into the abyss that I took, the easiest starting point for you would probably be either a YMDK ID75, or a KPRepublic XD75.
Those are both very similar to my board, but are commercially available, and involve a lot less work to build out.
They are both 75-key ortho boards, so the case is the same size as mine, but all the keys are single sized, arranged in a 5x15 grid.
Using that layout, as-is, would prevent you from having to laser cut a custom plate.
Having an actual PCB will prevent you from having to do all the hand-wiring I did, and will allow you to test out different switches, using the hotswap functionality.
Those boards won't give you the properly laid out numpad I have on mine, but would let you test out how well an ortho board would work for you, without investing too much work/time, only to find out you don't like it. >

I can only speak for myself, but a full size board is ridiculously big, unnecessary, and ugly.  I rarely use the numpad, so I have a separate one that I put away when not in use, and the majority of my boards are 75%, which retain all the functionality most people could need on a day to day basis. >

I tried going with the smaller keyboards, but I love the number pad too much. 
Before I started PC gaming, I cashiered for several years, the number pad was the most used part of my keyboard. So typing numbers feels most natural on the number pad for me; being raised on T9 texting as well might have something to do with it. >

I use a split 40% just for travel, because I hate the feel of switches on my laptop. >

Fair enough. I hate typing on laptops too. >

I have a 40% Ortho I like to use on occasion. One reason is that it's portable, I use it with my iPad when I'm out. I could buy a more compact membrane board but I like mechanical keyboards too much and it's portable enough. I find having the keys all super close together on a compact build lets me type faster in certain situations. I don't need to reach as far to hit numbers, but I do need to hold down extra keys which is the trade off, but it doesn't bother me. The biggest reason I built a 40% was to experiment with layers and to have something in my collection that felt unique to type on >

Just because you can't see the functions, doesn't mean they're not there. Assuming you're using a US ANSI keyboard, there is no dedicated exclamation point key, but instead you do Shift + 1. Small form factors are built on this concept, reducing hand movement by bringing everything closer to where your fingers already are by using layers.
I'd recommend giving this video a watch. It's a fantastic explanation on 40% form factor boards. >

There are a few people who are that short of space. I remember one guy from Hong Kong who couldn't physically fit a full-size keyboard on the tiny desk in his tiny flat. (I'll put a link in if I can find it.) And some people do carry their small keyboards between home and office. But largely it's just that people prefer a small keyboard, and are enabled to have one by the ability to program layers. 
Layers are vital to use a small keyboard. They allow one key to do multiple things - so on my Tipro keyboard I have a layer for symbols, a second for the numpad keys and a third for shortcuts and macros. So the key that started off as 8 can be F8, *, numpad 8 or four presses of Ctrl-= depending which layer key I'm holding down. With the ability to define layers, and the other things a firmware like QMK allows, it becomes less a question of how many physical keys you need and more about how much complexity you can keep in your head. 
The other thing I ought to mention is that some people find a small keyboard more comfortable to type on - they appreciate, say, being able to use cursor keys without moving their hands away from the letters. And smaller keyboards can be tilted and adjusted more easily, which some people find necessary. 
Oh yes, and some people need to fit other peripherals like graphics tablets or digitisers or spaceballs as well as the keyboard and mouse, so they run out of desk space that way... >

I appreciate the detailed and nuanced response. What's a Spaceball? (Other than the movie) >

I think the use cases for 40/60s isn’t the same as for typical office work. I have a 60 that I use for portability for LAN weekends but my daily driver is a 75.
At home I don’t actually need the additionals that a numpad provides and for anyone who’s just typing or gaming like me, chances are they don’t either. A smaller board is more pleasing aesthetically to me as well as allows my mouse hand to be closer than with a 100/110 board.
As for the reasoning behind spending all that money when something more basic accomplishes the same thing, well that’s just a question you can ask about any hobby really. I’m assuming for most here, it’s the feel, customizability, and labour of love that goes into the process. >

Thanks for the response! >

I went from full size Hardliner to ortholinear enthusiast. I did daily drive a Planck style JJ-40 (4x12 keys) for a while, then settled on a 5x12 Preonic. The JJ-40 was basically impossible to use for anyone but me. On the Planck, I have keycaps with legends and anyone can at least get some basic functionality out of it. It also feels more comfortable, I can get away with one layer less. >

Idk I wonder the same. I got a 75% to test myself. 
I miss my number pad. XD
You don’t realize how much you use something till it’s gone. 
The typing feeling is better though. Idk if $150 more expensive better but it’s a collectible hobby... right.