I believe that it's one of the options that admin can incorporate for us to show appreciation for a good post, advice, support, etc...I've seen it on other boards...Some people respond quite positively to good "kharma"; Becoming more active in the forum community...The opposite can also be true.
I've seen a few boards get a little kharma obsessed. I'm sure if it gets out of hand one way or the other it will be removed...