The elements for power are many: your body mass, striking speed, the distance to the target, range of motion, muscular strength, etc. Flexibility, among other elements in my opinion, is the king of all for the following reasons:

1. Flexibility allows you to relax a bit under stress. Knowing that you can reach a certain distance helps you be in calculated perspective, which in turn gives you advantage of clarity in thinking and prioritizing in strategy.
2. Flexibility provides you with wider options for targets to strike. You have a greater range of vital points to cover from wherever you are.
3. Flexibility leads you to the spontaneous mode of the mind-set, a sense of relaxed focus. You are able to wait and strike as a situation occurs.
4. Flexibility lessens your energy consumption.
5. Flexibility affords you to a have a greater range of motion, which is the essential portion of power production.