The Scotch pie roll is a finished pie put inside a bread roll, and the carbohydrate doubling is the whole proposition. A Scotch pie is a small, straight-sided double-crust pie of seasoned mutton or beef baked in a sturdy hot-water-crust shell with a lid set below the rim so the well on top can take gravy or beans. To put that into a soft floury roll is to wrap one engineered crust inside a second softer one. The defining fact is that this is not a filling between bread; it is a complete, self-contained, hand-sized object that already has its own pastry wall, and the roll is added on top of it for heft, for grip, and for the very Scottish logic that a hot pie is improved by being easier to hold and harder to finish. The pie is the sandwich; the roll is the second carb.
The craft is heat and managing two crusts that want different things. The pie has to go in hot so the fat in the mutton filling stays liquid and the pastry stays short rather than turning waxy and tight as it cools. The roll is soft and slightly absorbent so it yields around the rigid pie and soaks a little of the grease and any gravy without disintegrating, where a crusty roll would simply fight a shell that is already firm. The roll is usually split and the pie set in whole rather than crushed, because the pie's structure is the point and flattening it spills the filling and collapses the well. Butter on the roll bridges the salt of the pie to the wheat and seals the crumb against the gravy. Brown sauce inside, in a stripe, does the acid work against a fatty, peppery filling.
The variations are the rest of Scotland's baked-goods shelf met the same way. The bridie carries seasoned beef in a folded pastry; the macaroni pie and the mince pie swap the filling inside the same shell; the pie supper sets the pie against chips rather than bread. The Scotch pie eaten in plain sliced bread rather than a roll is the closer relative, a different carb framing of the same pie. Each of those deserves its own article rather than being crowded in here.