Interesting; I have no experience of snow shovels, but my interpretation from the picture is that the bit at the edge is reinforced by the vertical flange, so that no matter how far you wear away the blade, that will remain about where it started.

The middle bit OTOH, has the most give to it because it bends furthest under stress. Therefore its pressure aginst the substrate is least. The part between the middle and the edge has the highest friction because of the combination of moderate stiffness and lack of reinforcement.