By modifying the registry key you mentioned, you caused Windows to change the location of your personal Desktop folder.
Prior to changing the key, your Desktop was located under
C:\users\...\OneDrive\Desktop. This is called folder “redirection” and is what happens when one of your “special” user folders are moved away from their default location. Special user folders are Desktop, Documents, Videos, Pictures, etc. All of them can be redirected to new locations. OneDrive will do this for you during setup, if you allow it to. By redirecting your desktop to the OneDrive folder, OneDrive will sync it for you. By changing it back to its default location, the folder will no longer be synced. The desktop folder located under
C:\users\...\Desktop was simply a remnant or copy, and was not active - as you experienced for yourself.
Windows handles these special folders in a way that no matter where they are located, applications can easily find them. You also don’t have to worry about their underlying location, as a user, because all you have to do is click on the “Photos” folder in explorer to see the contents, regardless of where it is actually stored on the drive. This is why it is called folder redirection. You can also easily move these special user folders by simply dragging and dropping them and Windows will handle all the heavy lifting.
Windows handles the “Desktop” differently than others, in that it displays both the public desktop folder and your personal desktop folder together on your one “Desktop.” The public (called All User’s) desktop folder is displayed on every user’s Desktop. The personal desktop folder only displays on that particular user’s Desktop. The former is usually used for installed application shortcuts available to all users, and the latter is usually used for your own data and custom shortcuts.
Windows also has “Libraries,” and will often group your personal folder and the corresponding public folder together, in one view, even though they are stored in two different places.
Finally, the Public folder refers to a location that is accessible by all users on the system, and also network users on other computers - depending on your file sharing settings. Your personal folders are private and accessible only by your user account by default.