A:makes it possible to add and remove both the graphical shell and the management tools, making it simple to switch between a full server installation with a GUI and management tools and a Server Core installation.
Windows Server 2012 also introduced the ability to actually remove components from the disk, reducing the size of Windows on disk. Typically, all components are installed on disk in the WinSxS folder, even if the component isn't actually enabled--this allows Roles and Features to be installed without having to insert the Windows Server media.
When a Windows Server 2012 installation is installed in Server Core mode, the GUI and management tools aren't installed to disk in the WinSxS folder, which reduces the installation by around 4.3GB.
If you later want to add the GUI and management tools, an installation source is needed, such as the WinSxS folder of another server, the installation media (which could be on a share), or Windows Update (Windows Update has installation files for the Windows Server 2012 beta and release candidate in addition to RTM).