My use case: I like to have all my emails stored locally just in case some disaster happens with the copies in the cloud; I also get to have both personal and work email addresses, from different providers, in one organized and unified interface, and the same goes for tasks, calendars and contacts; and some features from big web clients are sometimes too nosy for my taste (suggested replies, pushing their calendar, messaging, tasks and contacts products, etc)

• You have an offline copy of your email records (and calendar and contacts) on your own computer without needing internet that no one can take away or lose.
• It helps if you regularly check multiple email accounts, such as personal and work, all in one place.
• Email integrates with some applications, such as Photos apps via the built-in share menu.
• Advanced features like email filters, mailing groups, etc, that may not be available on webmail.

I just don’t like to be in a browser for things that don’t have to be. At least not for things I use regularly, that don’t have to be. My computer has a window manager for a reason, I can move windows all over the place, and things like email I can keep always visible at work, or keep minimized and out of the way at home. But in a browser, the limited impact of tab groups aside, I’m pretty much stuck with a flat organization structure that can quickly become burdensome.

I also abhor notifications through the browser.

The message UI of K9 was changed and I think the app already implemented the auto-setup of accounts from thunderbird, otherwise its still the same app