Some subreddits require a certain level of karma to be able to post or reply to comments. I don't know if that was to help against bots or people who would make an account to avoid a ban or something. Other than that karma was just an ego boost for those who cared about such things.
It stopped about 2200 US eastern time for me...at least that's when I noticed it. I uninstalled and then deleted my Reddit account. I'm all in on the fediverse!