They manage the community, if you have unmoderated groups you get 4chan and similar which no-one wants to advertise on.

Also it incentivises moderating groups rather than just letting them be and doing the bare minimum.

I still don't like the idea but entirely for the reason that it adds financial incentives to demand moderators force groups to be advertiser friendly in more ways than just removing toxic content. If advertisers don't like something it will simply be neglected in comparison to the more advertising relevant ideas.