On my site I have a use case for people being given the privilege of managing group membership without being a member of the group themselves. I assumed this is how it worked and was surprised to see that someone I added as group manager is now also a group member.
The use case? We use groups to assign badges. And to give access to private categories. And the people responsible for doing this are not always group members.
Edit: having noticed this, I then removed some group owners only to realize they are still group members. Seems to me these should be managed independently.