If you haven't been organizing and discarding files as you go, it's likely your hard drive is stuffed with downloads, unwanted photos, file copies and other digital dust bunnies. This can reveal ...
The regular scan is just the warm-up for this button.
If Disk Cleanup is not working properly and does not remove Windows Update files, Temporary files, Windows.old folder, Internet files, etc. do this to fix the issue. Remove Temporary Files manually ...