Iirc #[5] has it as a feature since it's something he utilizes.

You can broadcast your note to only certain relays, maybe one that only you have access to. Think a bot reporting to you on something through nostr, you don't want everyone and their mom to see it so you have it broadcasted on a private relay. Then you decide you want to share it, you broadcast it to the other relays your connected to, so others can see it