Garage door panels are the largest part of your garage door mechanism. They also have to go through the actual movement while sliding on and off the tracks. This means that, even a minor incident during the motion could result into in a crack or a bend. It is often thought that the entire door needs to be fully replaced for maximum safety, however replacing just the cracked panels can also be a viable option.
Garage door panel cracks can be caused by the basketballs hitting them too many times. Panels in coastal areas will have their lifetime cut in about a half, because of the wet and salty winds, especially if they are made out of wood. Metal doors will suffer from more cracks in the paint too. Larger doors can get cracked by their own weight, so it is not advised to keep them in an open position for a long time.
If the panel is a little bent, rusty or in general can be fixed, aligned and put back in place, there is no need to replace it. However, if the panel is broken as a result of an accident or a really bad storm, it has to be replaced. Another important thing to make sure of is that the panel is matching to the rest of the panels not only for aesthetic reasons. The system is set to work with a certain weight, and so the panel has to be exactly the same weight as the others, or else you might have a broken spring on your hands pretty soon. The best way is to do it is to get a professional to match a new panel in the old one's place, and enjoy the peace of mind you get by having it properly done.