Why must recycled refrigerant be checked for air?

Air is considered a contamination in an A/C system. This is because it is a not a condensable gas.
This means that the type of pressure that the A/C can reach is not sufficient o condense the air from gas to liquid.
As a result, air will only be occupying space and displacing the refrigerant which will lead to a poor performance of the whole system and can even lead to total failure of the system.
This poor performance can be in terms of:
1- evaporator freeze-up
2- noise
3- reduced cooling performance
4- higher discharge pressure