When do I need to get a BLTouch upgrade? I've found many Youtube videos of makers upgrading to BLTouch sensor. 

I have a Creality CR-10S Pro, so far bed leveling hasn't been an issue (after some days of trial and error). 

My question is what are the problems/issues users need to face in order to need (or justify) an upgrade?
A touch or an inductive or capacitive sensor are useful when you cannot get a completely level bed which can be caused by a skew bed platform/heated plate or if the plate has a bend, large concave or convex area.

Even with such a sensor, you need to provide a bed that is as level (trammed) as you can get. Automatic Bed Levelling (ABL) is not magic, it is just a tool that can help out if you have a problematic bed surface. But, recent versions of Marlin have "manual bed levelling"; i.e. you can map the surface using your printer without a sensor and store that geometry in memory.