I'd say it's to counteract the "Eternal September" dynamics. As communities grow, they lose focus. The initial set of moderators may have done an excellent job

I'd say it's to counteract the "Eternal September" dynamics. As communities grow, they lose focus. The initial set of moderators may have done an excellent job at curation, but they can't scale to a growing community. The idea is then to have their judgement "flow" to other users.