Most of the HN bot accounts I see have a link-to-vibecoded-product in bio, and/or are trying to build up "organic" activity before a Show HN post for the same.

A less publicly-visible motive would be if they were building up accounts to use for paid-upvote schemes.

You can automate shilling to drive or at least influence opinion.

This is a venture capitalist driven community that attracts the sleaziest kind of spammers you could think of under the badge of growth hacking and networking. Besides this very obvious motivation to spam you have all kinds of nerds here eager to do it just because they can (on one of the most famous tech places where registration is made as easy as possible)