The idea with IMAP is multiple clients can work with your email - for example your desktop and your phone can both see the same messages and manipulate them, even offline.

Gmail basically is IMAP with a couple extras, and your desktop (via a browser) and your phone (via a dedicated app) can both see the same messages. Only the phone can work offline though, because there is little demand for a dedicated desktop email client, it's always via a browser. But Google could easily make such a thing if they wanted.