With a single click of the mouse button, you can examine any part of the entire image very fast by simply moving the loop around.
In Faststone the magnification, size and shape of the loop is user-defined in the settings. Thereafter, all that is required is a single click. Example, I have mine set to 100%, 500 pixels, round.
Note, the loop does not operate if the picture is in full screen mode (100%) as this defeats the purpose.
The advantage is speed. In PL, if you view at 100%, then the entire picture may not be on the screen and you would have to shift the picture with the navigation keys. The end result will be the same but the loop will be much faster. It would be similar to viewing at 100% then grabbing the picture with the mouse hand and dragging only much easier.
The loop should show exactly the same image as you will see if the picture is set to fit on screen - e.g.35%. So the picture is 35% whereas, the loop is 100% (or whatever you have it set to).
I would not have the loop show anything different.