It affected how easily your comments or posts could get upvoted, the more karma you had the more “trust” you had. Bot accounts were bought from people with high karma due to this reason, and sold further for a higher price after farming.
At some point the post/comment karma was changed to a fuzzy number, someone claimed your karma affected how it was calculated. Not sure if true.
Reading about how digg died, one reason was “front-page” bandits/groups. Basically people grouping up and getting paid to land posts on the front page. The more they did it, the more “trust” they had to hit front page again, similarly to high karma accounts on Reddit.