Penango (the company of the author) is a webmail company and so I would imagine that they wish to declare a mimetype so that they can default to supplying a MIME part that is text/markdown as the body of a message.
It's quite a nice solution... as you can believe that the text part is formatted reasonably well enough that you could present it as the text/plain part to external systems, and yet you know that you can also transform it to create the text/html part for other systems.
Whilst you only need to store the one part, the text/markdown.
It also gives you the ability to run a sanitizer after the markdown transformation, so that if you are a webmail host (as they are) you've got this really nice place to perform sanitization of what will be the email HTML before you send it to your web page.
