This technique is helpful with seeing and establishing nice crisp “grind lines” on knife blades with multiple planes. By using opposing angles on the different planes, light reflects off the surfaces differently and there is a more visible apex where the lines or scratches from sanding meet up. In the pictures the lines on flat part of the blade run parallel to the blade and the lines on the bevel run diagonally. Once it gets down to the finest grit or finishing, when very little material is being removed, the lines can all be run parallel for more of a standard looking satin finish or hand rubbed polish if desired.
The angles of the lines can be changed as the grit goes down. This also helps to see the scratches from previous grits as the surface is refined whether with single or multiple planes.