I can speak from experience. Limiting caloric intake will help you lose weight, but will do little to help you keep it off.
Cardio burns calories, and continues to burn calories after the session has ended. Even if the treadmill says that you burned 400 calories in 45 minutes, you continue to burn calories while your core body temperature returns to normal.
If done regularly, cardio boosts your metabolism. Cardio builds muscle. Every pound of muscle that you add allows you to burn an extra 70 calories per day. It also increases stamina - the amount of work that your muscles can do before becoming fatigued.
When I was obese, cardio helped limit my appetite. I felt queasy after exercise and the smell of fast food made me sick to my stomach.
Cardio strengthens the heart and improves the capacity of the lungs. A stronger heart and better oxygen delivery system helps to build muscles faster during weight training exercises. Muscle growth happens at a much slower pace if the proper nutrients can't get to the affected muscles in a timely manner.
Several years ago, I weighed 265 lbs at 5'9" and had a whopping 60% body fat! I could easily lose 25 lbs when I limited my calorie intake, but within a few weeks, it would be back. I started doing cardio and the results were slow, but sure. Within 3 years I lost 85 lbs, and dropped to 14% bf. My blood pressure dropped from 160/90 to 120/65, and my resting heart rate is 44bpm. My calf muscles grew from a non-existent state (13") to 19" and are very well defined. My quads, though not huge, are rock hard.
By doing cardio (about 1 hour per day), I can be sloppy in my eating habits and not gain fat. I just make sure that I take plenty of protein so that my body won't catabolize muscle for the protein it needs.
And, this is not a flame, but if you are only burning 400 calories in 45 minutes of cardio, you need to kick it up a notch. On a treadmill, a 200lb person will burn roughly 500 calories in 30 minutes at a pace of 6mph (a very slow run).