People will give you rep if they like your post. They may just click it, or they may leave a note. Some people will give you rep more to comment privately than because they like your post. Like alternative PM.
The more rep you get, the more little bars show up under your name. There is a bit of contention about it, that if you have a lot of rep, you just fit in.
The best explanation of Rep Points I have ever seen is along these lines(apologies, I don't remember who wrote this years ago): Everyone on NES is an a$$hole. Lots of Rep: Funny A$$hole. Not much rep: just an a$$hole.