...except in emergency situations. Starting the APU was one of the first things they did during the “Miracle on the Hudson” incident IIRC. It’s conceivable that some in-air emergency happening prior to the loss of communication would have required starting it as a checklist item.

EDIT: I just wanted to add that aside from electrical power, it provides bleed air necessary to start the engines, run de-icing (also run A/C packs, operate toilets, etc.) —Hence its usefulness in an emergency situation.

In some unusual scenarios the APU may be used during flight. If one of the engine's generators fails the APU might be used to help provide power, and below some altitude the APU in the 777 can provide additional bleed air.

In the 777 the left and right hydraulic systems are powered by the left and right engines, but the center hydraulic system has electric and pressurized air driven pumps that I believe can be run off the APU: http://www.flight-mechanic.com/wp-content/uploads/2017/11/12...

If MH370 lost engine power, maybe it was using the APU to keep the control surfaces active? Who knows. In that sort of situation the 777 would be a glider though.