Generally, providing all is well, the APU is not required while airborne. An example of its use would be a failure of an engine-driven generator. The B737 (and I presume the A320) has three, one on each engine, and one on the APU. Loss of one of the engine-driven ones would necessitate the APU being started. Single generator is generally an emergency procedure, albeit not major. In fact, I've actually flown sectors where an EDG was U/S on departure. The MEL (DDG) said this was acceptable, provided the APU and its generator were on line for the whole flight.