One of the keys to great actions shots is those pesky focus points that keep popping up in the wrong places on your SLR viewfinder. Taking control of these unruly buggers can improve your photography dramatically. Left to their own devices these unpredictable electronic scene analyzers often select something in the scene that is prominent, but not important to the photographer.
For example, you may be photographing a dramatic mountain scene with your children at play in the foreground. Unless you take specific action the camera may decide to focus on the mountains in the background while leaving the children out of focus. In this case it would probably be best to select the center focus point, aim the camera directly at the kids, depress the shutter button part way to lock focus and then recompose the scene for the final shot.
Some scenes are not so easy. Consider preparing to photograph wildlife in a similar scene that has not yet come into view. You can't use focus lock because you don't know exactly where the animal will be when you finally click the button. You can't just aim and shoot because the camera may select the background instead of the subject. The answer lies in the ability to manually select a focus point on the edge of the focusing system. Suppose you wish to shoot a vertical composition with the animal at the bottom and a beautiful background in the top portion of the frame. In this case, for a right handed photographer that is, select the focus point on the left side of the frame, rotate the camera to the vertical position and put that selection point on the animal when it comes into view. This technique also works well for sports images when the subject is not going to appear near the center of the frame. Notice on the image with this article that a focus point at the bottom of the frame near the fishermen has been chosen to assure that the fishermen aren't blurry.
Manual focus points are also necessary in closeups using long zoom lenses. In the case of pet and wildlife photography, the camera will often select the part of the animal that is closest and you will have a beautiful sharp image of a nose and a blurry animal. In this case, compose the scene, select the focus point closest to the eyes then recompose with that focus point directly on the eyes and shoot the image.
Don't wait until you are in the field to explore your camera's focusing system. Consult your camera manual and practice using the buttons until you can do it in your sleep. Practice selecting and using each focus point and practice returning the camera to it's auto selection mode. In a real life sports situation you may need to rapidly switch back and forth between focusing modes and you don't want to miss a bunch of shots while trying to figure it out.