There's a bit more to it than just their visual organization. In Discord, a user only needs to join a single community to access all of that community's rooms (they don't have to manually join each one to have it in their feed).

The admins of that community can then seamlessly create or delete rooms within that community (the users don't need to do anything for those changes to be seen and applied on their end), and can independently adjust what the base requirements are to view, enter, or interact with each room, and then give an individual granular permissions of what rooms are visible within that community.