The benifits of long barrels on early flintlock era shotguns were claimed to be,
* Longer burn time for poorer grades of black powder. (shoots further)
* Better shot patterns. ( Odd, I would have thought more bore scrubbing).
* Longer sight radius.
* Better suited for use with a bayonet. (You were part of the militia)

With modern powders and chokes I would think barrel length is a matter of personal preference.