but if you can expose a connection that goes to your personal relay people can message you directly
if you have a pc that is on most of the time it can act as your personal mailbox, and not only can people put messages to you, they can get your messages from you that relate to them also, if it allows them access when you follow them
public relays could literally store only profiles and relay lists and follow and mute lists and the rest could be all users own relays