It's a system that mirrors the merit system from bitcointalk, except that the karma you get doesn't affect your rank. It's useful because people can easily identify those who are useful to the community, or at least viewed as useful by the community. It may get used for something else later on, but who knows. It's a good thing overall. Oh, getting negative karma really impacts you badly as well, so it's important to never get dinged by making shitposts everywhere. Some bounties wont even accept people with negative karma.