In most flash use, the flash happens at the start of the 'shutter open' time. If your subject is moving, you'd find that, for example, a dancer is 'frozen' in one point and then the motion blur is gathered for the following movement. Think of a runner - it's more effective to have the runner at the front of the blur - leaving a trail - than at the back - which looks like he is 'pushing' the motion blur ahead and running into it. Rear Curtain sync will allow the camera to capture a little motion blur and
then freeze the action.
Similarly, think of a car's lights - you want a trail of light behind the car, not for the car to run into.....
this page seems to describe it reasonably well.