What other role does the APU fulfill aside from providing electrical power?

The Auxiliary Power Unit (APU) serves multiple critical functions for the aircraft, and one of its primary roles, aside from generating electrical power, is providing pneumatic power for engine starts. The APU operates by supplying compressed air necessary to start the main engines, particularly when the aircraft is on the ground and the engines are not yet operating.

When the APU is engaged, it produces pneumatic pressure that is routed to the engines, enabling them to start by providing the requisite airflow to their air starter motors. This feature is essential, especially when external ground power is not available or practical. It contributes significantly to the aircraft's independence while on the ground.

The other options imply roles that the APU does not fulfill in its designed function. Unlike backup hydraulic pumps or fire suppression systems, the APU's operation focuses on supporting the engines and providing electrical and pneumatic power, making the correct answer a reflection of its operational capabilities.