Other than being able to make general e-mail more secure, it is also possible to lodge either an entire message (in encrypted format for privacy), or a hash of a message on the blockchain. There are various reasons why this is important. Among them are the verifiability that a message was sent, who sent it, who received it, and who endorsed it (a third party).
When transferring assets it may be important to register the reasons and the associated transactions (contract numbers, invoice numbers, third party quality certifications etc.) alongside the actual transfer transaction in order to establish auditability and avoid disputes.