The good morning exercise is similar to a roman deadlift as it targets the hamstrings, glutes and lower back. The upper back will also be targeted isometrically (muscle contraction with no change in length of that muscle) as the bar is placed on the shoulders. As the good morning is mechanically similar to the squat, it could potenitally be used to improve squatting strength.
A common problem for people in the squat is losing upper back tightness, which can result in the barbell rolling and/or the upper back rounding. As there is an excessive forward lean/hip hinge in the good morning, a lifter is required to have a good grip on the barbell and upper back tightness. If the barbell is to roll on the back during a good morning, the loss of grip may feel exaggerated and the barbell may feel like it’s going to fall off the lifter’s back. As such, the good morning may be a good accessory to train barbell grip and upper back tightness for squatting.