I don't know the specifics of the 747 mass characteristics, but you are correct in saying that a good chunk of the mass is the plane itself. Not the cargo. But

I don't know the specifics of the 747 mass characteristics, but you are correct in saying that a good chunk of the mass is the plane itself. Not the cargo. But of the plane's mass, a huge amount of that is just fuel. Whenever they take on fuel, they measure it in thousands of pounds (in the US at least). So by reducing the cargo weight, you reduce the fuel weight as well because you can takeoff with less fuel. In other words, removing a pound of cargo can reduce the total weight of the plane by several pounds. For extremely short trips this would be irrelevant, because fuel costs are dominated by takeoff, landing, and waiting for clearance for those two things.