Nostr has the benefits of centralised networks for discoverability, federated networks for communities, and private individual web site for p2p and archival purposes.

As I have said in other replies to this post, read up on the outbox model. Global relays are useful, but are interchangeable and totally replaceable. As soon as you've connected with someone you can retrieve their updates, because you know their preferred relays, and can query them directly.

And there are incentives to running a global or community relay. Read up on Zaps. With Nostr, you can give real value via the lightning network, and it is built into the protocol. This allows you to charge for usage if you so desire. And then there's all the other reasons why people run community web sites or global services.