why steal and destroy the 000 GAL car?

They bought a wrecked Challenger. (the 000 GAL car)
They stole one (supposedly identical to it) from LAX airport. (In the movie, the stolen cars' front bumper and grille are different fron the wrecked cars': This is a continuity error, perhaps a deliberate error by the filmmakers)
They swapped the engine, transmission, and components (every part of the car that has the VIN on it.) from the wreck to the stolen car. (So the stolen car would "disappear" with the wreck's VIN on it.)
They sold the stolen car, with the wreck's VIN on it.
They destroyed what was left of the wreck.
They had to steal the stolen car back from Prince Chrysler-Plymouth when the insurance adjuster (who had handled the total loss on the OOOGAL car) saw the "wrecked car" (in perfect condition, with the 000 GAL plates on it) on the lot for sale. If they didn't, they would've been investigated for insurance fraud. Destroying the stolen car was necessary to get rid of the evidence.

But, really they didn't do this for the movie. The filmmakers just want audiences to think they did. All that mechanical work and VIN swapping wouldn't be done for a movie. It's just a plot line.

This is getting less likely with each passing year, more changes to cars mean less interchangeability between model years. And if you were in the business of stealing and reselling stolen cars, wouldn't all the labor costs involved with switchovers eat up all your profits?