broadcast will push an event to relays you're writing to.
example: I have a private relay to archive my notes (and other stuff) which I can connect to my phone over tailscale. if I make a post when I'm not connected to relay, I can connect after the fact and hit "broadcast" so the relay will have the event.
as for delete event, that would require collaboration of all relays that store that event to all delete it, which could be difficult to do.