A confession: I sometimes edit the gmail URLs by hand. In particular, I'm often using multiple google accounts and find that after restarting the browser (or something) a tab's account changes to something else than what I wanted. So I like having the option to just change the user id - the zero based integer in gmail URLs ("https://mail.google.com/mail/u/0/#inbox"). Also, in other contexts, such as google sign in, this id appears as the authuser GET param.