Why are trucks, truck tractors, and buses required to be equipped with spring brakes (or an equivalent mechanical device) for parking and emergency braking?

AAMVA CDL Manual, Section 5.1.14 (Spring Brakes), p. 5-3; 49 CFR 383.111(b)(5)(i) (automatic fail-safe devices)

This fail-safe design means a total loss of air pressure does not leave a heavy vehicle with no way to stop. Drivers need to understand that spring brakes apply automatically on air loss, which explains why a sudden air leak causes an abrupt, uncontrolled stop rather than simply 'no brakes.'