Activating Windows is essential for the proper functioning of your system. If you don't activate Windows upon reinstalling it, you will not be able to use many of Windows features, and there will be a more significant threat to your PC's security. You won't be able to use Windows Defender and won't get any security updates or updates in general. Microsoft constantly works on fixing minor bugs and issues with your system through updates, which will not be available if you don't activate Windows. You also won't be able to use Windows Hello and won't be able to customize many aspects of your operating system. A free or unactivated version also prohibits you from installing MS Office and related applications. On top of this, you'll be constantly reminded to activate Windows.