On Reddit, I think a lot of them monetize it. But generally speaking it’s a rewarding thing to feel like you’re building something greater than yourself.
Pretend that you moderate a food subreddit. And people in that sub talk a lot about fries. Specially classical potato fries and the "new" yucca fries. Often with heavy debates, but often just mentions.
Then you have ACME Yucca Inc., a brand of yucca fries, contacts you. And they offer you the following deal: "promote our yucca fries in your sub and we'll give you money." You accept it, and bend the rules of your community to selectively promote yucca fries and ACME over Wonka Potato Co. and potato fries. Stuff like this:
Alice says "eeew, potato fries are disgusting shit, go drink some bleach!". You let Alice do it. And if someone calls you out, you say "Alice is just expressing her opinion!".
Bob says "eeew, yucca fries are awful, you have poor taste." You ban Bob for rudeness. "We snoos shall not stand against this sort of hateful comment!"
Someone posts a bag of ACME yucca fries. You do nothing.
Someone posts a bag of Wonka potato fries. You remove the post as spam then report the poster upstream to the admins, so the poster gets shadowbanned.
Some user posts a rather bad recipe for homemade yucca fries. You let him do it, but then you go on your alt account and say "why bother? Just buy some from ACME"
Some user posts a rather good recipe for homemade potato fries. You remove the post because the submitter missed a comma, or another shitty reason.
It is totally fucked up. And the worst part is that it's damn hard to detect: the individual actions usually sound reasonable, you'll only notice that something is off either when it's too late (the sub circlejerks around yucca fries) or if you somehow know which mod did what, and follow his mod actions across a certain period of time.
On further hindsight, I should've picked another example. Because now I'm craving some yucca fries, too. With bacon, grana/parmesan, chives. And a beer. (They're a common bar snack where I live.)