Bobby, the pilot needs to control the transponder at all times. The transponder has four digits. Those digits run from zero to seven. Air traffic control has a finite number of codes that can be assigned. It is a little unusual to change codes during a flight, but not too unusual. I have had as many as seven code changes during one flight. ATC’s computer gets ‘full’ and as I transition from one airspace to another, that local controller asks me to change codes.
Transponders have a four position switch: Off, Standby, On and Altitude (for reporting your altitude to ATC). The pilot needs control because the altitude encoding function might have a problem. He still wants to be identified by ATC, but he has to sacrifice the altitude encoding capability.
And finally, you need to control the power to ANY piece of electronics from the cockpit, either with a switch or a circuit breaker, in case of a malfunction leads to arcing, or overheating and thus an inflight fire.
Great info! Thanks guys!