It is important so that the diffusion distance is shortened so gas exchange occurs more efficient.
As you may already know, lungs are used for gas exchange, and the alveoli is the surface for gas exchange. Gas exchange is where carbon dioxide from the blood capillaries (on the alveoli) are diffused into the alveoli, while the new oxygen that we breathed in are diffused to the capillaries.
Since the whole has exchange process relays on diffusion, so diffusion must happened fast in order to increase the efficiency. Therefore, the alveoli is only one cell thick so carbon dioxide and oxygen has a shorter distance to diffuse through so more of these gas can be exchanged.