First let us understand what are artery and Veins
An artery carries rich oxygenated blood from heart to all parts of the body and veins carry blood Low in oxygen for re-oxygenation.
Now the question is why do we have valves in our veins and the answer is , valves help blood flow in One Direction . Valves come into action when veins carries blood from all parts of the body towards heart.
So if valves are absent in veins it would make blood flow in bi-direction and that is blood can flow backwards and never reach the heart for the oxygenation.