Adding to broadcast guarantees that Amethyst will send to that relay. Otherwise it would just follow outbox/inbox of each user/event tagged in the post or the desired relay for nip-28 chats, ephemeral chats, NIP-72 communities, nip-29 relay groups, Livestreams or ephemeral chats. Each of these have a different rule on where to send the post. Adding to broadcast will make sure that broadcasting relay also receives it.