Originally Posted by Apache
If you are replacing your existing drive with another you can clone the old drive onto the new drive. this makes an exact copy of the old drive. this saves you from having to reinstall all of your software, including the OS.
The original reason was to reformat: (We will assume that) Windows has become damaged (Malware/Virus/bad luck). Cloning preserves the damage. Cloning is good for saving a 'about to fail, but has not...yet) drive. It is also used to put testbed systems back to a know state after an experiment went south