In the traditional Windows environment, the user manages the lifetime of his applications. He launches the application, and it continues to run until he decides to close it. The problem with this model is that applications continue to drain system resources, including memory and CPU, even when they are not in the foreground. This impacts the performance of the application the user is using as well as drains the battery faster when the device is not plugged in.
It is easy to demonstrate this traditional behavior in the desktop mode of Windows 8. If you don’t have a video file to use, browse to the following website that contains all of the video sessions from the Microsoft //BUILD event that launched Windows 8: