I moderated a mid sized sub for a while. Around 100k users. It was a hobby I was into and I figured may as well moderate because I was spending a lot of time on the sub anyways. It also let me put together some community events which were always fun. Once it stopped being fun and started feeling like a job, I left. I never really thought about it as doing free work for reddit and more helping community building for a hobby I had. People do it for all sorts of reasons. The "power mods" are really the issue.