During startup, they prevent the driver plate voltage from rising too high before the shunt regulator begins functioning. Once the amp is running, the are out of the signal path. (They also prevent some collateral damage if an error has been made during construction.) What's being protected is the PNP power transistor in the C4S, which is rated for 375 volts maximum. Semiconductors, unlike tubes, have no sense of humor about high voltages. If we could get higher voltage rated transistors, we would not bother with them, and the other amps have lower voltages and less need for the protection.
The technical term for this kind of design is "kludge".
