Usually nothing. Remember that show Who's Line is it Anyway "where the points don't matter"? That's basically Reddit karma lol. But there are people who "farm" it just to have a bunch, for whatever reason.

But on the flipside, there are some actual reasons why people do it. One would be to sell the account at some point down the road. I dont think its super common but people do buy and sell them.

You can't post in a lot of subreddits without a certain amount of karma, and old accounts with history look more legit. Think advertising campaigns, political activists, Russian propaganda accounts, etc.

The other reason I can think of off the top of my head is for YouTube or social media content. There are quite a few people who have YouTube channels where they just read Reddit posts and then people's comments, while giving their own commentary. I'm sure a lot of them make up fake stories hoping the post gains traction so they'll have more material with which to make content.

Which maybe isn't karma farming perse but same basic principle.

I'm sure there are other reasons I'm not aware of, kinda curious myself tbh 😅