It's just an added modification to make the forum a little more fun & interactive. Say you ask a question on diet, I'd like to know who to listen to. If someone with a negative reputation post's he's got it for a reason. I'd rather listen to someone with a higher reputation as they've earned it from posting something helpfull and it helps you distinguish who the decent people are and who the total *text deleted**text deleted*s are.
Edit: Another mod you could use is the "Thank" mod or w/e. Let's you thank people's posts.