Not everybody has the money for the same contraption. At the same way, it allows your eye to handle fast objects so you get used to fast things moving fast. Also, you normally do speed bags when you're tired as heck because it forces your arms up. This translates to having the ability to raise your arms up when you're tired in a fight. Raising your arms can defend you better than your arms down. It's the same way with doing the jump rope after a workout. It forces you to dig deep when you're already tired.