Cardio, by it's very nature is associated with positive heart and lung function. Without proper heart and lung function, your body does not use oxygen efficiently. Growth is much more difficult if your muscles aren't getting enough oxygen when you place them under a load.
Oh, and of course there is the fat burning aspect too. But, in my mind the other effects are much more important. Plus, if you're getting your fat burning from a bottle, you may actually be doing damage to your heart and lungs. No one gives a shit about this stuff when they're 20. But, trust me, when you're 41 like me YOU'LL CARE.