They do that to test the catapault...
and to blow off some steam. I've seen some photos of the cars fitted with fire extinguishers to simulate smoke.
I don't know if it's true, but on old carrier sailor once told me that if they could get the deck angle high enough, the catapault is powerful enough to shoot a car several miles. Considering that it can accelerate an F-14 weighing over 60,000 pounds from 0 to 150+ MPH in less than 2 seconds, that's a lot of power.