The use case for this is that you want to leave your gmail/youtube/etc all logged in, but not let Google track you all over the internet. My primary use for this feature is this specific use case. I created a "Google Spyware" container and configured Google-related things to open in there. If I go to google.com outside of that container, I'm a non-logged-in user. If I open it in the container I AM logged in, so I get all the convenience of staying logged in without the downside of all the surveillance. Of course there are other means to track me, but this is a big step in the right direction.