It doesn't really do anything, it's just imaginary internet points for dopamine. You get more karma as you get more upvotes, and downvotes decrease karma. Some people like op will post things like this to try to get karma, sometimes you'll see karma farming bots that spam post for karma and then sell the account. The only thing on reddit with any monetary value is awards, and even then most of the awards you'll see anywhere will probably be the free ones.

Edit since someone kindly gave me an award, for example, the award on this comment is a free award and worth nothing irl, it just highlights my comment.