The reason air is introduced into the wastewater is to introduce oxygen into the water and generate higher bacterial growth rates.
In fact, one can think of septic tanks as the opposite of aeration tanks, because septic tanks work to reduce agitation and rely on the natural settling of solid wastes to the top and bottom of the tank creating a middle liquid layer.
One might think encouraging bacteria to grow in aeration tanks may be counterproductive, but the bacteria aid in the digestion of the organic wastes contained in the wastewater.