The air gap controls the point of contact for the arms. If this is too small, the arms can not roll far enough past center to apply sufficient force to the pressure plate. A gap which is too large will cause the arms to tip further and be more aggressive, but the arms have limited travel and will eventually reach a point where they coil bind the dynamic spring and will not apply additional force the clutch. Additionally, arms too close or too far away will not apply at the desired RPM either. Maintaining this gap within the specs will insure consistent function of the clutch and help prevent damage to the unit.