In muzzle loaders with real black powder and BP substitutes, a longer barrel can effectively use more powder and provide slightly more velocity than a shorter one, thus a flatter trajectory. Also, most people fine a longer barrel easier to shoot well with (less wobble). With iron sights, a longer barrel provides a more precise sighting plane.
A short barrel, even a 21 or 22-inch carbine length will provide all of the velocity needed for hunting loads out to 200 yards, and carries a lot easier.
I love my little Omega X7 with its 24-inch barrel when I'm sneaking through the woods for a few hours. But sitting on a stand, I always take one of my long barreled Hawkens or Renegades.