Answer
It is important for the lungs to have a large surface area, because this allows for efficient oxygen absorption by the oxygen-depleted blood in the walls of the lungs.
The large surface area is possible because the inside of the lungs is like the surface of a sponge. As compared to a smooth surface, this spongy design has much more surface exposed to incoming air.
Work Step by Step
This is discussed on page 238. This is similar to the reasoning behind the elephant's large ears.
