Chicken breasts come in different thicknesses. If you try to cook them as is, they will cook unevenly. The thinner part will cook too fast and will be tough, and the thicker part will most likely be undercooked. Pounding the breast also tenderizes the meat, making it more tender. These days chicken breasts are getting larger and larger, and you can get two fillets from each breast. Depending on how thick they are, I have even sliced them into thirds. This not only makes your chicken go further, but it also makes it much easier to pound them out.