The difference between backup/mirror and move

I understand what you are getting at.  OneDrive is more of a file-syncing service than a backup service.

When OneDrive is activated, your files are moved to the OneDrive folder at C:\Users\username\OneDrive. Chances are, your Desktop, Documents, and Pictures folders have also moved to OneDrive.  When that happens, your Documents, for example, relocate to C:\Users\username\OneDrive\Documents.  You will no longer find your documents at C:\Users\username\Documents.

To ensure your OneDrive files are always available on your device, select "Always keep on this device." However, if you use OneDrive on multiple devices, it can be advantageous to keep your files cloud-only to ensure you have the latest versions and avoid older versions overwriting them.