Well, think of the psychological aspect of facing a shotgun in close combat. You don't necessarily have to kill you enemy but severely injure him or scare him enough, to turn tail & run.
Actually, the shotgun was carried by many mounted guerrillas & the effect of 2 barrels of buckshot backed up by 12 rounds or more of either .36 or ,44 caliber rounds from sidearms & that could put the fear of God of those on the receiving end in any battle. You get your enemy to fire his single round & while he is reloading, charge him with those 2 barrels of buckshot & it sure will rattle him while he is trying to load that single shot rifle.
Also, remember that many Confederate soldiers furnished their own weapons at first & a shotgun was probably all they had.
Having worked in law enforcement for 25 years, I can attest to the fact that the shotgun was the weapon of choice over others. There is nothing more attention getting than to hear a Remington model 870 being racked up.