Looking just at the two examples on the main page, it's an implementation of the SMTP protocol so you can either send mail directly to recipients' servers, or run a mail server.
Sending mail directly is often useful when you need to deliver a message to a lot of recipients. IIRC, this doesn't always work, because many servers reject messages unless they can reverse resolve the ip to the sender's domain.
A server for receiving messages is basic infrastructure. The reason for using a simple solution with clear source code, instead of established daemons, like postfix, is that you can tune it to implement filters or notifications.
As an example: some time ago I had the idea of putting a bayesian spam filter directly in the server code, eliminating the problem of false positives: the mail is either accepted or rejected, in which case the sender gets to know that the message has not arrived its recipient, instead of silently trashing it as spam.