I think it goes back to the original nostr days before outbox, ditto et all where the idea of nostr was to connect to a single relay at a time. That the relay was a specific community. I would have a relay for friends, a relay for family, that joe rogan would have a joe rogan relay. That notes wouldn't move accross the network but would be morw siloed into a group. I always liked that idea.
Discussion
That's the Fediverse model, which we know isn't the best because content can be censored or disappeared. I like specific relays for specific content or people, for community specific relays, but I want that content to propagate too.