Fastmail's web client is absurdly excellent compared to Gmail. It's as if, instead of copying all of the bogus stuff Google piled into Gmail, they just made a really good version of webmail the way it was in 2003. Instead of scrapping webmail and starting over, they evolved it into something better. The result is fast, clear, easy to use, and no-nonsense. It's as if all the features that bloat Gmail at this point are just there to solve problems caused by Gmail's bloated feature set. You really don't appreciate how bad it's gotten until you switch.
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.