Main function of valve is to prevent back flow of blood. This is because blood pressure in veins is very low (compared to arteries). There is no need of valves in the arteries because the blood pressure usually varies between 70-110 mm Hg. Imagine you have just opened a tap with full flow in a washroom and you applied your hand at the outflow. That is just 50-60 mm Hg (in normal circumstances).