When you bend forward with a rounded spine, especially when you are holding weights in your hands or on your shoulders it can be very dangerous to the spine. In the rounded position, the forces acting on the spine increase as the forward lean and the rounded position of the spine increase.
Thus, when handling loads that are in front of you even as when you are lifting a weight off the floor, you should maintain the spine in its slightly arched position and learn how to bend correctly from the hips. This action is especially important for athletes who must do a great amount of bending in almost all sports.
In addition to learning to maintain the lumbar spine in its neutral, slightly arched position, the good morning exercise provides a strong active stretch of the hamstring gluteus maximus muscles. The muscles undergo a strong eccentric contraction which then turns to concentric when you rise up to the standing position.
Many coaches recommend doing the deadlift instead of the good morning to accomplish the same thing. However, the effects are not the same and there are disadvantages to the deadlift. For example, to do the deadlift you must start with what can be considered the ending position in the good morning.
In this bent over position you place a great deal stress on the lower back muscles. If the arch is not maintained at this time you can easily injure the lower back. To alleviate some of the forces you bend the legs so that in essence you are doing a squat and trunk raise.
Because of this you do not get an effective stretch of the hamstrings and gluteus maximus muscles. In the good morning you are better able to handle the weight on your shoulders and control how far down you go by while still maintaining an arch in the lower back.
This is why the good morning is preferred to the deadlift for athletes, both for developing stability of the lower back and for stretching and strengthening the hamstring and gluteus maximus muscles. The deadlift is a powerlifting event and should be reserved for powerlifters.
For more information see Biomechanics and Kinesiology of Exercise.