Panning increases image sharpness with moving targets and can also help in conveying movement/speed by setting the target off against a blurred background as it allows a slower shutter speed to freeze the image compared to simply holding the camera fixed. Panning also allows you to get in closer to the subject with less area arround than simply standing still. Concider if you will a photograph of a car passing by if you stand still and wait for the car to arrive at the point where he is sufficiently large in your image then you have only a fraction of a second to capture the image, difficult. You are asking alot of the autofocus system to be ready and sharp and you need a fast shutter speed to get a crisp image as the car races past.
Concider the alternative approach of panning you have more time to position your target in the image as it comes towards you, the autofocus system can lock onto the target and trackit as you pan ensuring an accurate focus, the relative speed between you and the car is lower as you now have rotational speed to compensate. This results in more consistant sharp images.
Remember start panning early so you match the speed of the target, get a smooth motion and place the target in your image.