“Clean installs are useful if you want to start anew, or if you don’t have access to the old operating system anymore. If you follow Microsoft’s instructions, you would have to install the old operating system first before running the upgrade. To get a clean install, you’d have to install Windows 10 anew after that upgrade process. A user on Reddit discovered a method that improves that process significantly. Basically, it enables you to clean install Windows 10 directly provided that you still have access to the previous version of Windows.
Learn more, read comments, at ghacks.net.