Federation/Discovery for name servers
Username <-> ID mapping is useful for easy contact discovery.
As the character '@' is not a valid character for usernames it can be used for seamless integration of multiple name servers.
firstname.lastname@example.org -> lookup user1's ID on ns.jami.net email@example.com -> lookup user2's ID on ns.ring.cx
An even better solution would be to look up name servers through SRV-records in DNS, so
firstname.lastname@example.org -> get name server from SRV record: ns.jami.net -> lookup user1's ID on ns.jami.net
This enables organisations to easily set up their own (read-only) name servers and re-use eMail-Adresses as usernames.
However, lookup behaviour has to be defined, documented and implemented consistently across clients.