They popularized tags instead of folders. Tags I get - I can toss emails into "todo" + "personal" + etc and remove it from other views, without deleting it, and without using up more space. Many local apps still struggle to support this smoothly, though it's gradually improving. Same goes for server-side support, e.g. on Fastmail a duplicate in another folder costs duplicated storage.

Other than that, dunno. Desktop email clients have basically always been faster to use IMO, and setup has been borderline trivial ever since imap/smtp discovery strategies became common.

Also Thunderbird and Enigmail/PGP etc had been fairly solid for a long time.

> Many local apps still struggle to support this smoothly

Makes sense as the protocol they use to talk to the server doesn’t support tags.

Moving a message in a folder in Thunderbird makes a duplicate?

Most Thunderbird / desktop tag implementations are done by just copying the message to multiple folders (which achieves nearly identical behavior, which is great. It's just not usually optimized UX). And most servers don't deduplicate messages globally (I'm not sure why, tbh. probably there's some rational-but-depressing reason).

No, but you're moving it. Can you have the same message in three folders?

Ok I see what you mean, thanks

I can read email at home, at work, on my phone, on my tablet, on any other device that I can access.

My hard disk crashes? No problem, everything is in the "cloud". My computer crashes? I buy a new one and am up and running in no time without installing too much.

As can I using IMAP. None of my mail is stored on my local disk (and hasn't been since 1995).

Yes. You can always access webmail anyway.