Messages can be included in transactions in either encrypted, unencrypted, or hex messaging forms. No XEM needs to be sent in order for a message to be sent on the XEM network.
This can be used for secure communication to any XEM address, as well as blockchain based applications.
2 XEM/16 Characters - Unencrypted
Encrypted strings will vary.
Communication: Send messages securely (or not) to an address.
Identification: Send a message to someone to prove that you own a certain address.
Apostille: NEM Notary system that allows you to prove that a document existed in a certain form at a certain time.
Emercoin Copy: Emercoin is another well known and used coin that uses messaging as it's backend (though it's messaging system is closed-ish). Nearly anything that Emercoin does can be ported over to the NEM blockchain.