There seems to be a misunderstanding—doesn’t content in community relays also exist in other relays?
Here’s how I envision the scenario: I post something about my cat to my write relays. Later, I either republish this post to a cat-topic relay, or the operator of the cat-topic relay notices my post, likes it, and adds it to their relay.
Community relays are just a way to provide users with a place to discover content and people they might find interesting.