The main purpose is that over time people will build up reputations, so if you see someone with a load of positive Karma, then they are most likely respected and appreciated by the other members. If someone has negative karma, then they're most likely an idiot (or an admin or mod who has upset an idiot).
Please use the positive Karma function to "reward" people if they've helped you. You may use the negative one too, but please use it sparingly.
[Edit - we've since removed the negative Karma function as it wasn't popular with the BE members. Also it was abused too often.]