Kicks and punches have to have the power of the whole body (the reaching and turning power from the elbow, knee, hips) behind them.
This can be compared with baseball batting. You have to bring the bat (your fist, leg) to the ball (the opponent) at just the right place and at just the right time, and direct the entire weight of your body through that point and moment of impact. So it is very important to save your energy for that crucial moment.