Why do veins have valves?

Why do only veins have valves and not arteries?

Assertion(A): Arteries do not collapse, but veins collapse when they are cut through. Reason (R): Arteries do not have valves whereas veins have valves.

Why do veins have thin walls as compared to arteries ?

Assertion. Veins have valves which open away from the heart. Reason. Veins distribute blood to the various parts of the body.

Which one of the following does not have valves ?

Veins have valves which allow blood to flow only in one direction. Arteries do not have valves. Yet the blood flows in one direction only. Can you explain why?