I can see the use case for gmail at least. I tried to access web interface from India and it loaded like for 2 solid minutes before showing up completely unresponsive. I could have had it 10 times faster with a dedicated IMAP client.

Familiarity, better integration in the desktop, generally many more options (including extensions) than web versions, UI better adapted to a desktop computer, better at managing multiple accounts, are my reasons. I like Betterbird personally.

People have their preferences for UI and UX. I use Aerc because I like modal editing (ie being able to write my emails in vim) and keyboard nav. Using a desktop email client rather than webmail client from a provider gives me that freedom.

Besides, I don't actually have a webmail client I can use lol. I host my own email and host the IMAP server but I don't host a web interface.

I much prefer a client for usability reasons. My email provider has a poor web ui. I guess I'd need to change my email address to get round it. I tried the google web enail which was also bad. But google never care about UX. It also needed to refresh a web page on each click, where the client app is instant.

I can work offline.

Ultimately Email is old technology, all the web frontends just get in the way more or less.

I use an email host that has roadmapped switching their frontend to one I don't really like, so figured I'd get ahead of the curve and switch to a client that was open source and compatible with the typical standards — so I could learn it and never have to deal with another client again.

Ended up using Thunderbird, even for my old inboxes at the typical web companies

One client, all my emails in one spot, don't have to deal with stupid UX changes being forced on users.