[Alpo]

That wouldn't really be a single shot, since it's a double barrel. But the purpose is to give you a choice, when hunting. If the rabbit is running, you have a better chance of getting it with a shotgun, but if it's sitting still, head-shot with a 22 does not leave little pieces of shot in the meat.

Sometimes you want a shotgun and you're carrying a rifle. Sometimes you want a rifle and you are carrying a shotgun. With a combination gun, you have both.

Plus, if you live somewhere that limits the amount of guns you can have, with just one firearm, you have two different guns.