You can check their post history? Karma doesn't tell you anything, really. Mine went up tenfold one day just because I replied to what ended up as the top post in a top thread in a much bigger sub than those I normally post in. Some people spend all their time in big subs making short, smart remarks that get a lot of karma, others spend their time in enemy territory battling people they disagree with. Some toxic people have a lot of karma because they hang out in toxic subs.

The problem to be solved is how to order threads. Old skool bulletin boards just bump the most recently replied one to the top. Which works well on an old skool bulletin board as long as it isn't too large, but very badly on a big site where a few big active threads can drown out all the others.

I don't know what the solution is. But the numbers don't mean anything without checking the context. Karma is useful for ordering threads/comments, and giving users a bit of dopamine when they get some attention. But there (probably) are better ways to do it.

The thing is, high karma on Reddit doesn't mean someone has a history of thoughtful engagement. Just as often, if not more, it means someone whose well timed with zingers on popular posts.

And incentivising that kind of take-down behaviour actually creates toxic communities.

This is why it’s useful at the account level. It’s also useful at the post level in order to build a sorting algorithm which raises the most engaging/important/interesting submissions to the top. Within a community it is important to help define what that community is - irrelevant and low effort content is suppressed and relevant/high-effort gets boosted. Moderators can enforce this by just removing and pinning too, but that’s almost always too unilateral, and the voting system is generally better because it’s expected that then you get a representation of how people in that community feel about it. It’s a good system.