It's sorta had to describe in text, at least for me. But imagine a spinning disc in front of you that you want to stop for your hands. Would you place both hands on either side and press, using as much of your hand as possible or just squeeze the outer perimeter with your fingers? Using your entire hand to cover as much area as possible provides more friction and distributes heat over a greater area as well. Using just your fingers on the outside will either burn them or you'll be unable to hold on.

In relation to brakes you want pads that cover the full rotor, which can't happen if the calipers aren't selected or mounted properly for the application.