Question

why cars have bumpers that can be during crash

Answer 1

they are to save cars and passengers from big injury or damage. they are based on second law of motion. according to second law ' if time is increased then the force os decreases or we can aay that force is inversely proportional to time '.they work like this
when the car crashes with something then the bumpers increase the time to decrease the force of striking. this is same as a player catches ball and moves his hand backward. this is also done to decrease the force. in it the catcher increases time. now if time is increased then the force of striking will get decreased.

Answer 2

the bumpers absorb some of the shock, impact, force.  They get damaged and bent. so some of the impulse is absorbed by them.  they are like cushion.