Also if currently you are probably running Windows 8 on a virtual machine or luckily on a computer, there is not any doubt that this new operating system and especially the metro-style interface is dedicated to touch enabled devices like tablets. The plans of Microsoft infact include the new WOA keywork where the acronym stands for Windows On ARM that is the aim of making available this interface on a wide set of mobile devices that currently embrace this successful processor architecture.
Running on a tablet does not only imply a different input interface like the touch screen, but also it requires a careful use of system resources that are not always large on this kind of device. This is the reason why metro-style applications have an application lifecycle that is mostly similar to the Windows Phone than of classical desktop apps.
As a user, when you start a desktop application, you are exactly aware of the boundaries of its life. You click the icon and the application starts, then you use it and finally you hit the close button and the application closes. There are nothing behind this common process but you, as the final user, are responsible to decide how many applications runs at the same time so the management of system resources is completely up to you. Also if the previous sentence oversimplifies the matter, since behind the scenes the operating system manages system resources in an optimal way, if you continue to open a number of apps, at a given point you reach the maximum of resources available and you get an error.