The final stuff has to get to the customer. Now you can argue some agricultural products don't have manufacturing, but processing may be involved - but we'd be splitting hairs. Specialization is the key to our economic vitality. The creation of surpluses allowed civilization to trade and lifted us from a hunting and gathering to an agrarian economy. Specialization allowed for mass production.
Specialization also drives costs out of the system.
Let's just say a company chooses to move its own raw materials - it needs transportation, drivers, docks, handlers, claims and customs agents, etc. All of these costs and inherent direct and indirect labor costs are absorbed by a manufacturer who is in the business of making pipes, not transportation. Now it becomes the middle-man's job to drive cost out of the system. Specialization is the engine that is driving most companies away from vertical integration. Logistics and Distribution is the key to the miracle of our capitalist system.