Why is it important to use wheel chocks when parking a trailer that does not have spring brakes?

AAMVA CDL Manual, Section 6.2.8, p. 6-6

This is a real safety gap drivers can overlook: assuming the trailer brake is 'set' for parking the way a tractor's spring brake is. Chocking prevents a rollaway after the stored air inevitably bleeds down, which can happen hours after the driver has walked away from the vehicle.