Read original post by Shawn Kendrot at Visually Located
When writing a Windows Phone app you might need to prompt the user with login information. Maybe you want to know who in the family is using the app, or the most likely case is that you are trying to make an app that uses web services. When you want to give the user a login screen, you want this to be the first page that they see the first time they open the app. When they have already entered login information you don’t want to show the login screen and instead take them to the main page.