Most of us either ignore or find it too boring to cool down or perform static stretches post work-out. Now consider this, you have put your body in increased state of activity during your exercise causing the blood vessels to expand, bringing more blood into the legs and feet. When you stop exercising suddenly without taking time to cool down, your heart rate slows abruptly and that blood can pool in your lower body, causing dizziness and even fainting.
Further, your muscles are warm post exercise, and stretching (static), can help a great deal in improving your flexibility, which in turn helps prevent injury. So stretch every major muscle group after a workout, holding each stretch for 10 to 20 seconds and breathing throughout.
So athletes, runners, fitness enthusiasts, remember to cool down after every session to come back meaner and stronger for your next work-out.