If phones were cars

I think it is also for safety. Because on the inside the crash is softer when the car crumples. When it bumps back because of stability it hits the persons on the inside harder. But thats's just an assumption^^

Correct. Think of it as punching a brick vs punching a pillow. The pillow “crumples" and dissipates the impact. The brick breaks you.

It's to do with increasing the time it takes for the car to come to a stop. It is proven by basic physics, for example Force = Mass * Acceleration. So if you have a car like an old Volvo, a solid brick, it may take you 0.2 seconds to come to a stop. Compare this to newer cars, which use crumple zones a lot and it may take you 0.8 to stop when you crash. This decreases the total force by 75%. As a result, people tend to be safer, even if you may have to replace the bumper or even whole car after relatively small crashes. For older cars, most people tend to think that just because the car's intact and didn't crush your legs, it's safer. In reality, internal bleeding is much more of an issue in cars built like bricks, so the 75% force reduction which I used as an example means your internal organs are moved around much less, making internal bleeding more unlikely and less severe.
Source: Basic physics, learnt it at school - Just google "crumple zones"