Newton's first law states that, An object won't resist motion, if it is in motion, unless any external force acted on it.
So, As car is in the motion, Your body will try to be in motion, and when you suddenly apply the brakes, Car would stop, but your body will still be in the motion, so Your head may collide with Hard dashboard which could lead to major injury.
When you wear seat-belts, it prevents you from that motion, & you stay stationary on your Car seat.
Hope this helps!