It's a bootstrapping mechanism. Mozilla is not going to get instant buy-in from every single domain owner, so one way to use Persona is for the website that wants to do Persona logins to rely on Mozilla's central trusted Persona server, which in turn relies on actual e-mail verification. The other way is to bypass Mozilla's trusted server and rely on the domain implementing Persona natively.
If you as a domain owner don't want mail to be involved / want to make life easier for your users, implement Persona. If you as a domain owner don't care about Persona but run an email server, then anyone with an email address at your domain can still use Persona.