Crumple zones yield during a crash, changing energy from the crash, into heat and sound, which in turn reduces the chances that the passengers in the car will be hurt. (Crumple Zones)

When used in crumple zones, lightweight plastic components can help absorb energy and save vehicle weight at the same time. (American Chemistry Council)

The downside to modern "soft design" crumple zones is that they allow more damage to occur to a vehicle even when the collision is relatively minor and of no major threat to the safety of the occupants, thereby increasing repair costs. (Autotropolis)