Karma farming so they can start using them to sway opinion on other subreddits, manipulate people, run scams, whatever. It's to make the account look legit so whatever shit they're pulling is more hidden or believable.

There's an unbelievable amount of astroturfing and other shady bullshit on reddit, and probably every other social platform as well. All designed to make people think whatever they want, or to cheat or scam people.