Copied from ehow.com.
The primordial pouch is a multi-functional flap. It provides a little extra protection against kicks, which are common during cat fights as a cat will try and rake with it's rear claws. In wild cats, the ancestors of domesticated felines, this pouch appears to be present to provide extra room in case the animal has the opportunity to eat a large meal and the stomach needs to expand. It's also been said that this stomach pouch allows the cat to bend and expand, allowing for faster running and higher jumping.