The "key" works only when you updated from a Windows7/8 installation with that key to Windows 10 once. It apparently saves a hardware id on their activation server. After that you can cleanly install Windows 10 and skip the key entry and it'll activate based on that hardware id.
What could be done if you change the hardware ? If you had a retail key, then you'd probably need to install Windows 7/8 and upgrade to 10 again to repeat the hardware id registering process, but I didn't notice anywhere if this was tested.
What could be done if you change the hardware ? If you had a retail key, then you'd probably need to install Windows 7/8 and upgrade to 10 again to repeat the hardware id registering process, but I didn't notice anywhere if this was tested.