A good cool down can help prevent dizzy spells after an intense workout. Often, the reason we get dizzy spells is that blood pools in the large muscle groups after they have been worked out, leaving the brain deprived of oxygen and blood. Allowing your body to cool down will promote blood flow away from those muscle groups and back to a normal circulatory pattern. Also, it will help clear out the lactic acid (which gives a burning sensation) from the area. Lastly, it will gradually and safely bring heart rate and blood pressure back to their normal levels.
Following a cool down, a brief stretching period will help reduce delayed onset muscle soreness and can help make the day following a tough workout a bit easier.