I think there's probably a bunch of different use cases, depending on how its setup.

The most simple use case would be having a "private" relay for only certain people, to maybe only talk about a certain subject. In this scenario, only certain keys could Read or Write to the relay.

Or, the relay could just be readable to anyone, but only certain npubs could write to it - almost like a "quality filter"

Or, the relay could capture only certain types of content. For example, I've seen one that only displays short notes under a certain character limit. And another one that only shows posts from news bots.

I think there is not a single way to use this, tbh.