A lot of the time it's nice if the server can send mail on its own without relying on a plugin. With ssmtp, mail will work even when WordPress doesn't, and it means that Trellis doesn't require any plugins at all for basic functionality.
Which doesn't mean you can't use those plugins. They should work fine regardless of your ssmtp setup (correct me here, Roots team). There's even a Mailgun plugin out there that'll let you send via the Mailgun API rather than SMTP!