Yea I like to rebroadcast notes I think are important to ensure they exist on even more relays. It simply pushes that existing note/event out to your connected relays to make it visible to more users.

A note is just a packet of JSON (sort of) that is cryptographically signed. It can exist anywhere - when it’s first published it only exists in the publishers relays and maybe a local cache. Rebroadcasting sends a copy of it to any relays you are connected to that might not have it - if they already have it they will just ignore. Some relays also sync amongst themselves. It’s almost easier to think of a nostr event and its presence on a relay as two completely distinct things.