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Desire for power, authority, importance, respect, I'd imagine.

Cause then they can ban whoever they want that doesn't agree with them. Also, I'm sure that some of the powermods get paid to promote posts on their subreddit.

spend enough time in a (virtual) location and you might want to help improve the place/keep it tidy, or at least help the current owners/maintainers because you enjoy spending time there.

Or it can be just some ego/power trip.

For better or worse a lot of us have been passionate about the sub-communities we were involved in and we thought that our contributions were valued by the site.