You login once, you're set for years. Combined inbox from all your accounts so that you can see everything in one place. You only need to familiarise yourself with one interface.

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)

As someone who uses neomutt for mail, I may be an outlier. I like to feel like I have control over my email without having to worry about my internet connection. Also, I really, really hate the lag of web clients.

• 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.

For me, it's a matter of taste and philosophy. IMAP and POP3 are like ActivityPub, a public and widely-adopted protocol standard for their purpose. It represents my ownership of my email.

There's no unnecessary tracking or metrics while I draft my mail. I don't need Microsoft and Google's advertisements. If they decide to suddenly purge my almost 20 years worth of electronic mail, then I still have a backup copy.

Thunderbird is a really useful client too. You can even participate in matrix chatrooms through it.