That way, if your google account goes away (I guess we all agree that administrative account lockout is way more likely than google actually losing data) and you need an email RIGHT NOW, you would just configure any imap client to use the daemon provided by the backup application (localhost:143)
Or it could bundle some web based mail client and then open a browser.
It's impractical to reupload the whole backup to gmail before being able to access your mail.
But now you've got a background process running on a privileged port, yikes! (It's got to be a daemon that starts automatically, because "non-geeks" will not understand why they have to start another program before accessing their mail in Thunderbird.)