In lunge, the torso remains upright whereas you will forward lean your body in squat which you may stress your lower back if not done properly. So, lung is “back friendly”. Lunge focuses more in butt muscles , especially the largest one called gluteus maximus. The greater range of motion in lunge also improves flexibility , especially the movement in raising your legs. Flexibility will enable you to develop more efficient running motion. You achieve more power from the push off. By improving your ability to jump and run, you gain the muscle from functional as well as aesthetic perspective.