I would opine that the smoothing of the muzzle or crown by the use of emery cloth (a method I also used myself in my early days as a gunsmith apprentice)... probably had more use than just cosmetics...
Before WW2, smokeless powder cartridges usually had corrosive priming, and a polished muzzle would be much less likely to "capture" poweder/primer debris from the muzzle blast and the smooth metal would lessen the chance of a corroded crown... The same would hold true for a polished and blued muzzle, but the crown would eventually be white from the blast effects of shooting... a smooth white muzzle would always look good after cleaning, but a blued muzzle that was in the process of being removed by shooting blast would eventually look terrible... hence the smooth white muzzle on firearms.
I can remember polishing the muzzle end of my blued 1911A1 barrel so that the size of the bore would be contrasted against the blue of the bushing and slide... so that if it were pointed at someone who was a threat, that they could ponder the size hole it would make in them.
