Medicine balls build core trunk strength and joint integrity. You may ask why is this important?
Core trunk strength is the key to most athletic moves (i.e. golf swing, throwing a baseball, martial arts, blocking and tackling in football), the problem is the body is a chain and is only as strong as its weakest link. The trunk is generally the weakest link. All sports trainers now recognize core strength training as the logical starting point for an effective training program.
Joint integrity is the other vital benefit of Medicine Ball training. Often athletes over strengthen muscles at the expense of joint strength and no matter how strong your muscles are you are only functionally as strong as your joints. You see this often in injuries to shoulders, elbows, hips, and knees. Medicine Ball training is designed to strengthen these areas and allows you to more fully access your muscle strength. You also become more resistant to injuries.