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.
I think you’re going to begin to see a lot of that on Reddit. I overwrote and delete my ~10 years of comments and posts before deleting my account. I imagine a significant number of others have/will too.