The driveshaft is a long rod or tube used to carry rotational force from a vehicle's engine to the wheels. Driveshafts vary in size, configuration and features. One variation is a hollow driveshaft, used for several reasons.
The engine would have to power the rotation of a solid piece of metal. With a hollow driveshaft, there is less mass for the engine to push and the force it produces is able to accelerate the drive train faster.
Making a hollow driveshaft uses less material than a solid one. With some good engineering, manufacturers can make hollow driveshafts with the same structural integrity as their solid counterparts. The saved material lowers the cost.
Because of the reduced rotational inertia, the engine can get the same push with less work. This translates into fuel efficiency. With less material to push, it takes less fuel to get the same acceleration.