Well not everyone uses #nostr. Just as an example nostr:npub108pv4cg5ag52nq082kd5leu9ffrn2gdg6g4xdwatn73y36uzplmq9uyev6 runs a bridge that lets nostr users interact with ActivityPub users.
If you’re only using your own relay, and not reading or writing to any other relays, then you probably don’t benefit from that. (Unless of course you get those bridges to read & write from your relay).