I've never made a barrel but I have done some blacksmithing, and it seems to me like it would be awful hard to make a straight barrel by hand - one hammer mark a hair deeper than the rest (and in forging anything there is always that one mark that doesn't want to file out) means that you have to reduce every flat along its whole length. With a swamped barrel you can just alter the profile slightly to accommodate without having to reduce the width of the whole barrel.
Also, that flair at the muzzle should increase the moment of inertia there where it will do the most good, and make the gun hold a little steadier without greatly contributing to the overall weight of the barrel.
Regarding the subtle swamp found on some originals, I came up with a way of calculating the volume of a swamped barrel a year and a half ago while in the process of ordering a custom profile, and one of the things I discovered is that even a 1/32" here and there does make a difference in the overall weight of the barrel, at least on paper. Dunno how much of it would be noticeable in the hand, but I would discount the possibility that even a very subtle swamp makes a difference in how the gun handles.
