Windows 10 biggest change is the fact that your activation status is stored in the cloud, so that way you don't need your product key when you reinstall Windows 10. Every time you reinstall Windows 10, the device is activated automatically. Previously, every time you needed to install Windows, you needed the product key unless you had an UEFI-based laptop with Windows 8.1 pre-installed.
When you reinstall Windows 10, it will still ask your for your product key (twice, in fact) but you can go ahead and skip. Your device will still be activated because Windows 10 recognizes that it was once installed on the hardware once before.