Why do oil (fuel) Tankers have more than one Compartment instead of carrying all the quantity in a single one?
This is in order to create a baffle system. When transporting liquids, under acceleration and braking, inertia makes all of the fluid weight to act on one side of the container. Similarly, while cornering, the forces act sideways. Since such a characteristic poses as a safety risk as well as leads to quicker wear and tear of vehicle components such as the brakes, baffle system is used to break down the magnitude of forces.
Other than that, different compartments help to deliver to a wider variety of consumers. For example A may require lesser quantity of product than the full capacity of the container, and similarly with B and C and so on. Thus the container can carry the optimum load and deliver the same to A, B etc. as per their individual requirements.