Good question.
There are several things:
2) Enjoying the poems, limericks and stories written by the many talented writers who share their output here.
4) The opportunity to help other posters with info I may have. Teaching other people new things.
5) The opportunity to get info I need from other posters.
7) Hearing points of view that differ from my own.
8) Learning new stuff about topics I never followed.
11) The many halacha posts and discussions that allow me to expand my knowledge and do a bit of learning while on this site. I will sometimes follow up something read here with my own research, so additional learning also gets done based on posts here.