1) swamped barrels are the correct profile for rifles from certain time periods. Basically a flintlock rifle made in the colonies/states before 1790 would have a swamped barrel, although the profile might be less obvious than our current offerings.
2) swamped barrels are lighter while having nice thickness at the breech. That is good for balance, the architecture of a longrifle, and it's strong, too.
3) Swamped barrels look good in the right context.
4) The rear sight is already a bit higher than the front even if they are of equal height.
To me #1 is the most important.