The free upgrade to Windows 10 gets you a Digital License. That Digital License is stored on the Windows 10 Activation server along with the hardware hash of the PC it was issued to. Basically a digital fingerprint of your PC. 7 Home got you 10 Home, so if you install 10 Home, and just click "I don't have a key" on the screen that asks for a key, the install will continue and activate with the Digital License stored on the activation server. The key you were issued was a generic key that everybody with 10 Home gets with the free upgrade. You could type it in, but its easier to just skip entering a key.