It lets the community defend itself against spammers and other undesirables, and provides a low-cost/low-effort feedback mechanism. Other sites use reputation systems too - they have real benefits in terms of creating responsible behaviour.

I find it quite useful - if I don't have much time to read the rest posts, or comments to an interesting post, I'll only read those with high karma, so I appreciate the time-saving mechanism. It also creates an incentive for high-quality comments, which I appreciate too.