The lat pulldown works the latissimus dorsi, the largest muscle in the back, as well as your biceps, rear delts, rhomboids and traps. This movement gives you the opportunity to train many muscles at the same time, and develop overall strength in your back and upper body. These muscles play an important role in posture, as well as allowing you to develop pulling strength, and pull yourself up and over an object, like you would do in activities like rock climbing.