At the very least, I knew that if mail ended up in that inbox, it was usually important.
More to the point, Fastmail allowed me to use Mail.app on macOS and get away from Google's web UI which, despite best efforts on their part - read: none at all - never worked correctly on the desktop with regard to message deletion in IMAP clients.
There's a high 9's probability that it will arrive safely, but there's several ways that things can go wrong, simplest of which is that you are deleted by a misconfigured filter.
There are US ISPs (cough sbcglobal) that routinely throw away mail from some arbitrary IPs despite there being absolutely no SPAM blacklist entries for the IP, even if you are replying to a mail sent from one of their clients.
outlook.com once, (and maybe still does) blocked email from an entire C-block for some reason.
(edit - grammar)
And there's the core of all of this. SMTP is a best-effort protocol that does not guarantee delivery. Until someone comes up with something better, we're condemned to seeing "Help! My very important email disappeared in transit" threads on HN every several months for the rest of eternity.