crumple zones are designed into the structure of the car to collapse or crumple up in the event of an accident - if you look at your bonnet it would have folding lines in the middle - thats exactly where the bonnet becomes A shaped when you are in a crash - similarly the front of the car will crumple inwards to absorb the hit energy. If these are straightened out with heat - the car may look straight but in the next crash event the amount of energy required to crumple them will be less than half of the original. The next place inline is the passenger cell. Such incidents usually end with an annoucenment from a masjid and order of a pulao ki daig for the soyem of the person driving it.
These crumple zones are a one time use only piece - thats why such cars are scrapped in other countries because they cannot be insured to be road legal. The crumple zones can be replaced with new parts but other than bolt on parts like bonnet, bumper cover, replacing the unibody panels gets very very expensive. Sometimes costing more than the price of an old car.