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.
I have read online (but not verified myself) than using a Windows 7 or 8/8.1 key works. This might be true or it might be misinformation. The upgrade only took 10 minutes so I did it that way with the plan to always wipe and clean install after. I decided to just jump through the MS hoops of doing it 'their way' and it all seemed to work just fine.
According to Gabe Aul's tweet [1] it looks like they do associate the device id/hardware hash so there is no need for a product key when doing a clean install once upgraded.
OEM Windows 7 Home Premium (SLIC Cert)
OEM Windows 7 Home Premium (Sticker)
OEM Windows 8 Pro (Embedded in BIOS)
Retail Windows 8 Pro
None of them worked with a clean install, you have to manually upgrade your version of Windows, let the setup finish, and then you can do a clean install without introducing a key.
Some people have already confirmed that Windows takes a fingerprint of your hardware and saves it into their activation servers.
Honestly, I am concerned about the Retail Win8 key in case my motherboard breaks, but hopefully that won't happen in the next 5 years.
