Probably to get used to it. From one of my psychology classes many moons ago experiments were done to help people get over their fears by over exposing. For example, with balloons a person would be put in a room full of them and they would be popped one at a time and maybe a few at a time. But of course it would be different if you are expecting them or if somebody pops one behind your back. Not sure about air bags because they can be pretty loud.
I had this for some time after we had an attempted break in and my husband and I were both shot. Any noise, even a plastic water bottle popping as it warming made it jump. It was a reaction I hadn't expected.
For me, it was only time (2 years) that has calmed me.