Its original intent was to filter good vs. bad content. Prior to karma/voting systems, message boards were just a list of the most recent posts by anyone. With a voting system, people can decide what content best fits the community's purpose. If I post a dog image on a cat forum, people can downvote the post so newcomers aren't seeing dog pictures on a forum about cats. Without karma, you're relying entirely on moderators to manage that. It's basically crowd sourced moderation.

Karma has other issues for sure. It can be manipulated with bots. People tend to use it to say "I don't like this opinion" and not to say "this opinion is within the domain of this forum".

All of that being said, I believe karma systems should be hidden from the users. Jerboa is an Android app for Lemmy and it shows the karma count. I don't prefer that. I like being able to vote, but I don't want to feel the bias of "big number == good opinion". But I think karma is a good system for helping moderate the content that shows up in a forum. It's a democratic way of managing content. But it probably has room for improvement.

I think it was always the same psychology of making a number go up makes people get dopamine or something. Otherwise, it was a system to try and filter out bots used for astroturfing that I felt didn't really do a good job. There were always plenty of karma farming bots that would literally just copy and paste a different comment to create a fake post history.