The mill marks on a new barrel run vertically, scattering the light forward and backward. The scratches from draw-filing and sanding run longitudinally along the axis and scatter the light up and down and sideways. So not only does the barrel look nicer but it's also less likely for reflected light to interfere with sighting. If you sand after filing, don't go any finer than 220 or the bluing or browning won't "take" very well.