I had to answer this basic question today, so I guess others may have it as well, so I thought it’s worth sharing :)
If you had a chance to play with new Windows 8, you should have noticed that applications do not have a “close” button or even the menu item to exit the application. You still can close the application by pressing Alt + F4, or drag the title of the application to the bottom of the screen to exit it, but that’s not convenient, right?
The reason for that is that you don’t have to do that.
As I’ve illustrated earlier, in Windows 8 the “Metro” application doesn’t get any resources once it loses focus. So when you start another application, press Start button on your device or the keyboard, or browse back from your application – it takes up to five seconds to stop the execution of that application. It may still reside in memory, but only while that memory is not claimed by anyone else, so the performance of the computer doesn’t suffer.
The advantage of leaving the application “open” is that you can later switch back to the app, and you’ll see exactly the screen you were on when you’ve switched from that app. So you will not have to waste your time to navigate there. It’s like teleporting to the living room instead of opening all the doors of your house, navigating through the corridor… You get the point.
Even though it appears like your application is open somewhere in the background and “consumes resources” – it doesn’t. So that makes the gesture of closing your application an anachronism, a habit from the past which lost it’s reason.
NB: Of course, it doesn’t apply to desktop applications, the ones that have classic buttons to minimize, maximize and close the window and can be resized. These applications are not affected by WinRT and will continue to use resources unless you close them.