It's mostly people who keep getting banned or keep making new accounts for some reason that fall into this. Or bots posting to generate tons of link karma but never posted comments and never got comment karma. It's to prevent those who get banned or bots from being able to flood a sub.
/r/nfl was notorious for deleting posts from unknown users when new broke to repost it from a mod's account. Had nothing to do with ban evasion or bots, just leveraging the control they had to make sure they had all the biggest posts.
