Say we have a mint tin that we're measuring. We want to know the thickness of the metal, but because of the rolled edges, there's no easy way for us get a good measurement with the outer-pincers. However, we can use two measurements and subtract them! This is what digital calipers are good at.
Step measurements are taken by pressing the 'head' against the bottom of the tin and then lining up the offset movable side so that it presses against the top of the edge.