There's been discussion of a sort of relay pool. Kinda like a mining pool. Relay operators can be part of the pool. On a technical level, all relay operators are behind a centralized load balancer for a single domain name.

Its best to think of nostr as a reimplementation of RSS. Social media over RSS with cryptographically singed messages.

Put another way, its like seeing facebook, reddit, stacker news, twitter and more all in one home page. Each of these websites have different moderation policies, but adding another site to your feed, is easier than logging into and checking another website. So if your favorite content creator says "sorry guys we got banned from twitter, please follow us on minds" or something like that, instead of signing up to minds and remembering to log into it now and then, just add the feed that comes from minds, to your main feed and it'll get mixed in with your feeds from the other websites.