* To increase pulmonary compliance.
* To prevent atelectasis (collapse of the alveoli or atriums) at the end of expiration.
* To facilitate recruitment of collapsed airways.
Alveoli can be compared to gas in water, as the alveoli are wet and surround a central air space.  The surface tension acts at the air-water interface and tends to make the bubble smaller (by decreasing the surface area of the interface). The gas pressure (P) needed to keep an equilibrium between the collapsing force of surface tension (γ) and the expanding force of gas in an alveolus of radius r is expressed by the Young–Laplace equation: