I've been wondering about this for a couple of days. Having users-only instances could help with moderation and content visualization (think in beehaws.org pulling the plug on lemmy.world), while giving the freedom to different communities mods to treat their content as they want.

This would lend to three levels of administration/moderation.

1. At community level: Mods can let their communities be run as they want
2. At content-server level: By letting content-server admins run their instances with their own ethos. Letting people from certain users-only instances post, while other only read, and others are simply blocked.
3. At users level: By letting users-only server's admin to let their users access certain instances that are aligned with their own interest (no-porn instances, no-nsfw instances, etc.)

This can lead to a kind of meta-db, where instances can declare their ethos, and then be automatically peered, or automatically severed.

I think that the main benefit for this is that it's easier for newcomers to visualize. While having mixed instances removes redundancies, having this separation allows for more streamlined experience for the users.