Cardio exercises are MUST if you are trying to have a six pack or a flat stomach. Cardio exercises basically burn the excess fat on top of your tight muscles and help you expose the hard worked muscles underneath those layers of fat. You can do crunch after crunch, but if you don't burn the fat your muscles will never show.The heart rate and metabolism are basically increased by doing a cardio exercise and with proper trainning it is possible to burn fat, even hours after finishing your workout. These exercises are one third of the factors to have a flat stomach (the others being strength training with stomach exercises and dieting).
totally agree I find a great way on how to get flat stomach is to do high intensity interval training which continues to burn fat upto 24hrs after a cardio session. Give it a try it works wonders for my body fat.