When you die you can trade it for computer time.

Neurochemical feel good tool for contributing content to the platform

Build up enough of it and you could sell your account to spammers.

Mostly, it had no point, but to me it was durable evidence that people were interacting with my posts, even if they didn't feel the need to reply. But, IIRC, subreddits could limit posting/replying/voting based on karma levels, and I know Reddit did take into account your total karma when doing rate limiting.

Its social capital capital. On places like stack exchange, a person with high "karma" is an indicator that lots of people have found their answers helpful. Similar with reddit, but for posting good content.

It was a score. Like a game.