i agree with Neta, everyone has different motivations, but i see it as if you get information it's nice to give back, though not required.
my motivation is , not only do i (hopefully!!) help someone get the answers they need but whilst answering them it makes things clearer in you own head, althought you know certain things, untill you actually put it on paper or explain it to someone it never become completely clear.
so not only are you helping them you are helping yourself, and you don't have to answer questions that bore you, "how do i change a light bulb!?" just answer things you are curious about.
hope this helps and hope you will answer some of my questions!??? 