[infosechandbook]

> You can save XMPP account IDs in any mobile address book.

... while the rest of the XMPP account remains on the XMPP server, inaccessible. There is no benefit as you can also store other primary IDs like phone numbers in your address book.

And the rest comes down to "continue a conversation with someone somewhere else" isn't so easy but the same pain as with any other messaging system, minus you may use the same XMPP client, plus you may need to search a while until you find an XMPP server that comes with the fitting XEPs for your use cases.