On the face of it seems strange why anyone would want to moderate popular subreddits, just seems like a lot of unpaid mundane and never ending work. Younger me might of been excited to do it from the perceived status perhaps. I do wonder how many off-site deals are done with moderators and third parties. There’s a lot of traffic these people are able to steward and influence I imagine.
It's a little like picking up litter while out for a walk. Not exactly fun, but can be fulfilling in a way.