You may find it counterintuitive, but lower-body training is advantageous when you want to stimulate upper-body muscle growth. Leg workouts stimulate some of the largest muscles in your body, which helps create a metabolic state that is conducive to muscle-building. You can build muscle in your upper body without working out your legs, but working all the major muscles in your body will lead to faster and better results. And nobody wants to be told they looked like they skip leg day, right???