Among the identifiers a Windows PC carries, the MachineGuid is one of the most-used. It is a unique value created when Windows is installed, and it survives every update — which is exactly why so much software treats it as the PC's identity.
Where it sits and who reads it
The MachineGuid is stored in the Windows registry and is readable by any program without special privileges. Windows itself uses it; so do Microsoft Store apps, Office, OneDrive and many third-party tools. For an anti-cheat or a licensing system, a stable value like this is an obvious anchor.
Changing it safely
Because the MachineGuid is just a registry value, it can be changed — but doing it carelessly causes trouble. Software tied to it can demand reactivation if the value changes out from under it. That is why it should be changed as part of a coordinated HWID change, not edited in isolation, and always with a saved state you can roll back to. HWIDChanger handles it as part of the standard profile and keeps the previous value in your account.
One thing to check first
If your Windows or Office is activated, make sure that activation is linked to an account you control before changing identifiers. That way reactivation, if it is needed, is a sign-in rather than a hunt for a lost product key.