Why use different shutter speeds?

Shutter speeds determine the length of time that the shutter is open, and that the film or sensor receives light. If I wanted to freeze the motion of a speeding car for example, I would need to have the shutter open for a very short period of time. If I wanted to show that the car is moving, I would leave the shutter open for a bit longer. Many different effects can be achieved by changing the shutter speed.