It's just one important piece of the puzzle. Slow movement, which for a fit person could be jogging, is the base of the fitness pyramid. Doing one without the other, leaves you short of your potential. As for losing weight and fat, I agree that diet is 80% of success. It doesn't have to be all sprinting either. Do Tabatas with weights or exercise to compliment the sprinting. Just don't drop the slow, lower heart rate movement.