What is the purpose of delaying RF output after activating a transmitter's keying line to an external amplifier?
Why External amplifiers use relays to move the antenna and input paths between the transceiver and the amplifier's tubes or transistors. Those relays, especially mechanical ones, need a few milliseconds to fully close. If RF appears before the contacts settle, you get hot switching: arcing, pitted relay contacts, damaged amplifier input circuits, and possible SWR faults in the transceiver. A programmable TX delay in the transceiver holds the RF off until the amplifier has completed its transmit/receive switch.
Watch out The power supply idea is tempting, but amplifier supplies stay energized in standby; it is the antenna changeover relay, not the high voltage, that needs the settling time. Key clicks come from too-fast CW envelope shaping, a separate issue.
Relays first, RF second. The delay prevents "hot switching" of the amp's T/R relay.