It is used in the event of an electrical failure/ hydraulic failure, one of the two, its a small propeller that drops below the aircraft and gets the systems working again
It's basically a wind-driven generator. The idea is not new. Some of the earliest aircraft were fitted with a wind-driven fan (like a small propeller) to power the fuel pumps & electrical equipment.
PS. Here's a typical Hamilton Sundstrand RAT as fitted to some modern jet airliners like the A330/A340.
Yeah it's important. The onboard batteries cannot power the aircraft that long, so the electricity generated by the RAT is very important, aswell as the ability to power the hydraulic pumps so you can control the aircraft without having to be a bodybuilder.... :)
I sometimes do wonder how heavy it is to fly a large aircraft without hydraulic power steering Ive tried feeling the controls on a helicopter once with and without hydraulics, but imagine a large 737 and bigger? whew