because in many organisms like fish etc it is an organ for respiration the gills are supplied with capillaries so that the oxygen absorbed can diffuse into the blood it has large surface area so that more oxygen can be diffused in and more co2 can be removed
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If the surface area of the organism is small, then there is no problem with getting all the oxygen needed. If the surface area of the organism is large, therefore a special respiratory surface is needed. An example is lungs, gills. The ratio of surface area to volume in a small organism is greater than the ratio in a large organism.
Leaves are broad, thin and flat to provide a large surface area, to absorb sunlight for photosynthesis. :)
large area
The Alveoli
So it is more efficient.