Stance is vitally important for two reasons:
1. They provide a stable platform from which you can deliver powerful techniques.
2. They provide a stable platform from which you can move quickly and easily.
A "natural posture" is actually a form of stance and it's important to practice defending yourself from such positions. But there's a right and a wrong way to stand in these "natural" postures.
Without a firm root, your techniques will lose most of their power and speed. As one old karate master once told me, "Stance and stepping is one of the great secrets of karate but nobody practices them anymore..."
