Like Martin and Mark, for me, answering questions has multiple benefits:
- It reinforces the information in my own head
- It allows me to experiment, to find the best way to verbalize the information
- It gives me sort of a "group-quid-pro-quo" feeling; I feel like if I help out enough on a website or in a forum, I'm more likely to receive help later if I need to ask; alternately, if I ask for help and receive it, I'm more likely to feel indebted to the site and hang around to see what help I can offer.
Of course, having cool badges is nice too.