[jenny91]

The APU is not on during flight, so not a safety thing. It's just for providing power (and bleed air) on the ground (if there's no other power source). E.g. can't start up at airports without external power with the APU INOP.

[sterlind]

if you don't have a RAT you need the APU for emergency power if your engines fail in flight.

[addaon]

APUs have a common mode failure with main engines: lack of fuel. Are there designs out there which require emergency power after engine failure and don’t have a RAT?

[aivisol]

If I remember correctly it came very useful during US Airways Flight 1549 accident?