Being able to switch from macOS to Linux/Windows seamlessly.
Realistically next time I buy a new laptop it won't be heavily premeditated: the odds are that my old one will die and I'll need something that works that week. I don't want to end up buying another Apple just because I don't have time to switch mail clients.
Obviously a mail client only takes a few hours to set up, but it's not just the mail clients: I try to avoid anything Apple-specific for the same reason.
With the current incarnation of Thunderbird there is even a switch you can flip to make the email client offline, even if your PC is not. Believe it or not, that is an extremely valued feature here.
With synchronized folders and offline storage it makes composing, reading, and replying easy with outgoing queued until next time connected when out and about with a portable device.
Example: When it is in the middle of popping up suggestions as you compose, it suddenly seems to decide you aren't in the compose window any more and interprets anything you type as a keyboard shortcut (e.g. "n" for new message "e" for archive). Since I type rather fast, this happens to me all the time.