What additional function does the APU provide besides operating the gearbox-mounted generator and supplying bleed air?

The auxiliary power unit (APU) plays a crucial role in aviation systems, primarily providing power and bleed air when the main engines are not running. One of its additional key functions is to start the main engines. The APU generates the necessary power to operate the starter motors for the main engines, enabling them to begin generating thrust. This function is particularly important during ground operations when external power sources may not be available.

While other options mention functions like generating electrical power, controlling flight surfaces, or providing emergency oxygen, the specific advantage of the APU is its capability to initiate the startup of the main engines, thereby ensuring the aircraft can transition from ground to flight readiness efficiently. This vital role underscores the APU's importance in aircraft operations, especially in situations where immediate engine power is needed.