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 growth. 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.
Muscle growth occurs when you break down fibers through stress -- in this case heavy resistance training. Satellite cells rush in to repair those areas so the muscles become thicker and stronger. Specific hormones signal the activation of these satellite cells and the cells present within the muscles, thereby stimulating muscle growth. The more muscle mass stressed, the greater the release of these hormones. Because the legs, along with the glutes, contain some of the largest muscles in the body, heavy-resistance leg workouts do a lot to stimulate growth hormones and testosterone, which benefit muscle growth all over the body.
Not Just Legs
Leg workouts alone won't create a balanced, muscular body. The anabolic, or muscle-building, state that leg workouts help create is only valuable to your upper upper body if you are also breaking down those upper-body fibers through strength workouts. Squats can't replace presses and rows -- they augment these moves. A nutritious diet that includes a slight surplus of quality calories, plus adequate sleep, are also key in cultivating the ideal muscle-building environment.