Vasodilation is the process by which the blood vessels in the body relax and widen, allowing for greater blood flow. One of the main triggers of vasodilation is nitric oxide (NO).
Therefore, anything that increases nitric oxide production in the body will also increase vasodilation.
From an athlete's perspective, vasodilation allows for an increase in oxygen and nutrient delivery within the body, resulting in greater energy production for the muscles.
Vasodilation also helps improve removal from muscles of waste products like lactic acid and ammonia that contribute to muscle fatigue and failure.
Other areas that benefit from vasodilation are those leading to the skin to cool off the body. Ideally, you'd be in a cool environment so you don't waste too much blood flow to your skin, which is the reason why it became so popular to wear ice vests or take ice baths prior to important sports events. Read more here.
Vasoconstriction can also be useful, not in the vessels supplying the sports performance muscles but in the vessels supplying visceral organs for digestion.
This exactly what happens during maximum effort and why it is so difficult to digest solid foods during sport events you normally have no problems with in a regular workout. Vasodilation is not just beneficial for athletes but is also a powerful method for those seeking brain performance or easier sexual arousal.