The cool down has vital physiological functions. When your heart rate is increased from exercise and you suddenly stop your activity, the blood flow rapidly decreases, which can cause blood to pool in your feet.
Since blood carries oxygen to your brain, heart, and muscles, it is vital to decrease your exercise intensity slowly and keep the blood flowing evenly.
The cool down also allows your muscles to cool down slowly. Without a cool down period, muscles may cool down rapidly and tighten up. This can lead to muscle stiffness and soreness.
To properly cool down, decrease to a light intensity level until your breathing and heart rate have slowed. The cool down is also an excellent time to stretch.