There’s no drilling holes or measuring. You can hang a mirror exactly where you want because you don’t need to find a stud to hold the weight. You can reposition easily without big holes to fill.
Some mirrors are backed with card or hardboard. Mount the Attach-Its on the hardboard using longer screws if you can screw securely into the frame. Otherwise pad out Attach-Its so prongs can fully penetrate into the drywall.