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.
Once you cross the extremely small karma barrier needed to get your comments allowed/not be rate limited...whats the point? I use reddit for a lot of recommendations/research and I have _never_ checked the karma of a user after reading their comment.
Account says buy bitcoin - user checks history and sees 7 year old account with thousands of posts, many highly upvoted, so assumes source is sane and not astroturf.
That's my point though: how often do people do that check? I don't think I personally ever have. Maybe I'm just weird though? I tend to not just go with a single user/comment/thread, but rather try to find multiple different answers and gestalt from there, so individual trustworthiness is less important.
You have to remember that with spam, they're looking for gullible people, and the marginal cost of the spam activity is extremely low. To some extent, the Nigerian prince scam works not in spite of, but because it's nonsense.
there are definitely browser extensions that will call the API to check account age and highlight suspicious accounts, that's how "checking" generally happens except in egregious cases.
Karma on Reddit is used as a proxy for "this user makes good contributions". At smaller scales in the past this might have been a useful proxy but when a pithy joke on a huge subreddit can get thousands of upvotes it's not longer a good proxy. But since people still use it as a proxy astroturfers wants as much karma and activity as they can manage because it makes the account look more realistic.
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.
That's at least the explanation I've heard many times.