What primary function does the APU serve during starting of engines?

The APU, or Auxiliary Power Unit, is crucial during the engine starting process as it provides backup power and air. Specifically, during engine start, the APU generates pneumatic pressure, which is used to turn the engine's starter. This process facilitates the initiation of combustion within the engines. Moreover, the APU also supplies electrical power to essential systems while the main engines are not yet operational. This capability ensures that the aircraft's electrical and pneumatic requirements are met without relying on the engines themselves during startup, which is especially important in ground operations.