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.