Well mostly they are used for general trades here on aff such as requesting for advertisements, posters and reviews and other services provided by multitude of different shops here. You can bid for an ad too, and it will be promoted by the aff staffs themselves (havent personally tried this one bc it requires a thousand plus karma points bc of bidding). Changing your username requires karmas, also transferring a story to a co-author, you can also gift someone with it, and the likes. But for further explanation, you can check the FAQ page of aff, there is a section dedicatedly solely about karma points. ^^
