People who want to astroturf, from everything from products to politics (though honestly probably more for "products"). Reddit is well known for being one of the few places you can "trust" product reviews (that hasn't been fully true for a while now but it's still valuable). If a near-0 karma and/or new account says "I love X product" then they can sometimes be downvoted/called a shill but an account that's a couple years old and has decent karma? Looks like a real person.

That's at least the explanation I've heard many times.

Outside of buying accounts, some people literally just want control. For the same reason that people will spend countless hours moderating large subs for free. They want to push certain narratives on subs, so you'll see large accounts where all they post are generic filler distractions or inciteful links. It's not all super nefarious or political, often it's things like mindless fanboy wars, where they only post things that make their favorite companies look good.

For others, they do it because getting upvotes and reddit awards makes them feel good. They see themselves as community pillars. I suppose this kind of feeds into the above reason in some ways.

If you check /r/games today, you'll see one user is responsible for over half of the submissions. Such accounts are what I mean, where unless they have fine tuned a bot to such a degree, they're really just someone with a little bit too much time on their hands.

Did you see all those "where can I buy this flavor of snack in $location", or some weird anecdote involving a flavor of chips or similar. Tons of astroturfing on reddit.

I notice a lot of talking past each other in only tangentially related comments. Lately also here on hn.