Why do chillers using CFC-11 and HCFC-123 require purge units?

These refrigerants have higher boiling points than say R12 or R22. Thus when a machine using them operates the low side will run in a vacuum. So any low side leak points will suck in air rather than leak refrigerant out. A purge unit constantly samples the condenser gas and removes the air and water vapor which has been pulled in at these leaking points and periodically releases them to atmosphere while it condenses any refrigerant vapor picked up in the sample mix to a liquid and returns it to the machine.