"The purpose of an (APU) auxiliary power unit is to provide power to start the main engines. An airplane (APU) is a also use to keep the aircraft computer and accessories on while the engines are turned off. check this"
"It is the power generator. When the engines are running, like in a car power is generated. When they are turned off, to keep on the lights and avionics in the cockpit they run. Since so much electricity is used a battery is not practical for a long…"