I worked for a couple of years in the Tech Startup space not long ago and in little companies like that everybody does kinda work with everybody else, so I did work together with the Digital Marketing side too.

Anchored in what I learned there I have a feeling that Karma is often used as a sort of buy-in and gamification strategy.

On the first part (not sure if buy-in is the right expression but stay with me here), it gives people something that feels like a personal asset: you've put time into making posts and you got this "stuff" from it, which intellectually is just a number by emotionally is something that is "yours" and you got by putting time and work into it, and this "stuff" is non-transferable so you're less likely to leave because you don't want to loose it.

On the second part it's all part of a game loop to incentivise posting: you post, people read it, they like it, so you get karma, which feels good so you post some more to get more karma in turn resulting in more of the pleasure of recognition and that "score" going up. Mind you, up-votes also do work a lot like this (not the score part but the pleasure of social recognition) and karma just amps up that a bit, so this second part is not as important as the first. (Also note that this whole game-loop is why many social media sites don't have or removed down-votes - with only up-votes pretty much everybody no matter how shitty their content gets at least some of that sweet positive social-feedback, which feels good so they'll make more posts so there's more content on the site which attracts more people spending more time there, yielding more eyeball-hours for advertisers hence more $$$).

Karma does make sense in a purelly expert context to allow people to recognize those with somewhat more expertise (though it really doesn't measure that with a correlation of 1, as people get karma for sounding right, which is not the same as knowing what they're talking about), but in a system like in Reddit it doesn't work like that because one can gain far more karma from just saying something which is "popular" and "aligns with the groupthink" in some political-heavy sub or making interesting posts in the "relax" subs (say, posting jokes, memes, cat-pics) that you can by providing genuinelly knowledgeable expert advice on expert subs, as do it with a lot less effort, so people's karma doesn't really work well at showing expertise, unless maybe its per-sub.