A casual observer of karate may get the idea that power and strength are the key factors in a good technique, when in fact, they are not. Also, tightened muscles give his opponent some idea of what technique is coming, making it easier to defend against the attack. When you relax, your muscles are looser, making it easier to throw faster punches, kick higher, and react quicker to attacks. But, relaxation is also a state of mind. A martial artist must condition his body so that he is able to be relaxed when practicing or when it is necessary to defend himself.
Flexibility makes it possible to hit a target that's just out of the natural range of motion.
Properly stretched muscles can extend to their full range of motion, allowing them to make contact using maximum speed and maximum force. Flexibility is essential for maintaining good balance. Without flexibility, techniques won't be as sharp or as controlled and throwing multiple techniques will be nearly impossible. In karate, speed is of the essence. The power of any technique thrown depends upon the speed with which it is delivered, not just the mass of the deliverer.