This is because the Windows 10 activation status is now stored online, after you activate your copy for the first time the device will activate automatically, with no product key required. The installation will still ask for the key but what's new here is that you can skip this step. More discussion about the implications of this and several tips & tricks can be read over here.
When you upgrade from Windows 7 or Windows 8.1, the Windows 10 setup program checks your current activation status and reports the result to the activation servers. If you're "genuine" (that is, properly activated), the Windows activation server generates a Windows 10 license certificate and stores it in conjunction with your installation ID and the version you just activated (Home or Pro).
It didn't need a product key to do that activation. All it needed was the proof from the Software Licensing Manager utility that your underlying activation was legit.
You can now wipe that hard disk completely, boot from Windows 10 installation media, and install a squeaky clean copy.
The Setup program asks you to enter a product key, but in a major change from Windows 8 and 8.1, it allows you to skip entering that key.