Why are pat pending bricks rarer and treasured more than patented bricks? My mum and I share a love of LEGO, but she is a huge collector of LEGO with easily 100kg spread across the garage, her room and my nan's garage. 

When she gets time to sit down and sort through her immense collection, she's always more excited when she comes across a brick which has "pat pending" written on it. She has spent hours searching for pat pending bricks to complete an entire retro set out of this brick type. 

I want to know why two otherwise identical bricks can be so different in importance - a standard brick vs a pat pending one - to an avid collector of LEGO, when to me, they are otherwise the same and I'd probably mix them up by accident.
As I understand it, "Pat. Pend." bricks were made in the 1960's.

Later around 1970 they removed "Pat. Pend." by carving it out of the molds, so there is a lump in place of it. These are sometimes called "Pat. Pend. removed". Those ran until 1976-ish?

I don't think any of these bricks are much more valuable than the others. Old generic bricks aren't generally worth very much. If one wanted to have genuine 1970's pieces for personal reasons, I guess they might look for these markings.

I don't think all pieces had those markings though.