Works offline, has its own OS-level windows (so I don't have to hunt around for the tab, can close the browser but keep e-mail open or vice vers), multiple e-mail accounts under one application while keeping them entirely separated (I don't want to give one provider access to my mailboxes at another provider, and I don't want to have to use multiple different web UIs). Also a matter of habit.

Multiple accounts and GPG support

(1) Desktop clients used to be faster and more efficient than using a web interface, and probably still are in most cases. However, Gmail is impressively snappy so I don't think it applies in that case.

(2) You can do things with a desktop client that you can't do with a web interface. That includes sort by subject and sort by size.

(3) Desktop clients can have real folders, which Gmail doesn't.

(4) Works off-line. This is still useful, though not as useful as it used to be.

(5) Easy to backup. Easy to automate backup.