It's designed to manage software licensing, but Microsoft can also use it to identify individual users.
"Microsoft has confirmed that Windows has a previously unknown Global Device ID (GDID), a permanent, unique digital identifier assigned to Windows devices that can tie a user's actions to their device. This came to light when an alleged member of a notorious hacking group was caught at an airport after Microsoft handed over his GDID to the authorities.
Microsoft describes the GDID in a published complaint (via Windows Latest) as "a persistent, device-level identifier designed to uniquely identify an installation of a Windows operating system on a device, either a physical device (e.g., a mobile phone or laptop) or virtual machine, across certain Microsoft services and scenarios."
So when you first set up Windows, or register a new Windows device, Microsoft ties that installation's GDID to you. Anything that happens on that system can then be traced back to you, the individual."
