[rgbrenner]

That's for their Files product (which you can use to create a website): https://www.fastmail.com/help/files/website.html

They can't request a certificate unless you already pointed your domain to their servers. So if you're already doing that for HTTP, requesting an ssl cert so they can enable HTTPS on it is a perfectly valid request... especially when browsers are starting to mark all HTTP sites as insecure.