Advantages of Using Pre-Focus
Pre-focus is a term and technique practiced by professional photographers, and it is highly useful in action photography with lots of movement. If done right, you'll have an awesome shot. If done wrong, you'll either have a blurred frame, or even worse, an empty frame. Although it might sound technical, it's not. Anyone can do it.
You would pre-focus so that when incredible, breath taking action happens, you're there to hit the shutter on your camera can capture that image forever in perfect sharpness with everything in focus.
Predicting the Action
The advantage of pre-focusing is that you can predict the future. If you're going to shoot a shot of a motor cycle stunt man jumping through a ring of fire, then you only have one chance to do this. Since the ring of fire is a fixed point and not moving you should focus your camera on that distance so that when he does jump he'll be in focus. The only other factor that you need to control is your timing when you hit the shutter (you also want a faster shutter instead of a slower or your shot will suffer motion blur).
Since pre-focusing is a guessing game instead of a science you will not be right all the time. If you don't get the shot, don't be worried, something else will come along. The most breath taking shots that impress people are also the hardest to get.