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.
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This is discussed on page 238. This is similar to the reasoning behind the elephant's large ears.