Here is the reason.
Each shave causes small deformations in the edge. Looking down the length of the edge, the perfect "V" shape has tiny regions that fold over into many small "J" deflections toward both sides of the blade. With each shave those deflections get worse and over time will break off causing microchips.
Stropping bends those deflections back toward alignment with the edge and stropping before each shave minimizes the cumulative deterioration of the edge.