You should always wear your seat belt just in case the car comes to an abrupt stop. The seat belt will hold you in place so that your body does not continue moving when the car stops.

Newtons 1st law of motion states that the object will continue to move at its present speed and direction until an outside force acts upon it.

So unless the objects inside the car are restrained, they will continue moving at whatever speed the car is traveling at, even if the car is stopped by a crash.