The purpose is to rate the users.

Individual communities can set up guidelines, that if you have a new account under 6 months, and you have a negative overall karma, you’re banned from that community until a human can look through your post history to see if you should be unbanned. you’ll have to repost previously highly upvoted content to pump up your karma numbers, until you have a positive overall karma.

FTFY, I’d really prefer to leave that mistake of karma at Reddit instead of polluting Lemmy with it.

Lemmy karma-less method also drastically reduces the value of bot accounts to farm karma (for nefarious or advertising use before being banned).