-Probably because they're bored, or they try to use it for something (for what i dont' know, maybe to get some info on something? )
-For shits and gigles? some people just do it out of boredom.
-Most of the time they don't, if you talk about forum hacking, they always do, and of course they are.
-If you're talking about toribash accounts, probably for items and tc, as for other sites, probably for whatever virtual items they have there.
Getting hacked does suck, but no matter what site you go to there WILL be hacking.