The idea is that it gives a "fairer" representation of just how worthwhile a member is than their post count. By rating a users posts as good or bad you influence their level of karma. The more "good" reviews someone gets the more positive their karma will be. And vice versa for "bad" reviews.
So, if used wisely it should be possible (for example) to better represent someone who may only have posted a few dozens times but whose posts have been very useful ... from those who've posted a few thousand times but where their posts consist of "Yeah, I agree" or "ROFL", etc. :)
In time the intention is to allow karma to be used in the event system (assuming we implement it
Of course, like anything it could be abused ... although you have complete control over just who can rate posts in a forum.