What is the purpose of the safety valve installed in the supply (wet) tank?

The safety valve is a spring-loaded valve in the supply (wet) tank set to open at about 150 psi. Its job is to protect the air brake system from over-pressurization in case the governor fails to cut out the compressor at the normal pressure (around 125 psi). If you hear the safety valve releasing air during normal operation, the governor has failed and the system must be repaired before driving. The safety valve is not a normal control — it is an emergency relief device. The other functions described (releasing air at shutdown, bleeding for descent, applying spring brakes) are handled by other components in the system.