Leather is miraculous at polishing steel. The tiny pores of the leather grab at the rough burrs of steel on the edge of the razor, and force them back into alignment. By stropping the edge of the razor along leather repeatedly, you can re-align the edge to the nearly perfect state it should have been in before the shave. By using your strop to align your blade every time you shave, you keep your edge sharp for far longer, and therefore only have to sharpen it every few months rather than every few weeks.