I know, it wasn't initially clear to me either. nostr:npub18ams6ewn5aj2n3wt2qawzglx9mr4nzksxhvrdc4gzrecw7n5tvjqctp424 and nostr:npub1gcxzte5zlkncx26j68ez60fzkvtkm9e0vrwdcvsjakxf9mu9qewqlfnj5z had to break it down into simple / small words for me, and I'm still not sure I fully understand it (so take what I'm saying with a grain of salt).
Apparently, it works like this because the "Mailbox" model is a relatively new concept, and not all clients support it. General relays is the "old school" Nostr way.