In terms of impulse and momentum, why are air bags in automobiles a good idea

During a car crash, the car decelerates extremely rapidly, so inertia causes you to move forward quickly. If there wasn't an airbag to slow you down, you might fly out the windshield.

The air bag increases impulse by increasing the time during which a force acts on you (impulse=force*time). This decreases your forward momentum, so you'll have a better chance of surviving.