The idea behind "App Tabs" is that some web apps are more like applications than web pages. For example, Google Docs, Gmail, ...

Things you would want to have open all the time and they could potentially be chromeless since you don't navigate through them with back/forward. Sort of like built in Prism. If, say, you wanted Gmail open all the time without chrome you could drag it to the left and pin it thus making it an App Tab.