http://xmr.nowherejezfoltodf4jiyl6r56jnzintap5vyjlia7fkirfsnfizflqd.onion/opsec/serversideencryption/index.html
Bob encrypts his message using PGP , and no matter where he sends it (over mail, over discord, over IRC, XMPP, facebook, etc.) only Alice will be able to decrypt the message. In short, Bob uses PGP because he doesn't trust the platform on which he wishes to talk to Alice.