Veins have valves to prevent blood from flowing backwards and pooling, whereas arteries pump blood at higher pressures, which naturally prevents backflow. Veins need valves to keep blood flowing in one direction because the flow is less constant; the flow in arteries is constant and requires artery walls to be more flexible and strong enough to accommodate the high pressures.
The valves in veins allow blood flow in one direction while blocking any counter-flow. These valves assist the body in sending blood in the correct direction; for example, blood flow must work against gravity to move blood from your legs back toward your heart. The valves ensure success, as they keep blood moving forward in the veins.
While these valves work quite efficiently when intact, they tend to break down during aging or when people are inactive for long periods of time. These breakdowns allow blood to pool where the valves are not working effectively. Such pooling leads to varicose veins.
Rather than requiring valves to regulate blood flow, arteries require flexible walls that can withstand great pressure to accommodate the flow. The constant motion of blood in arteries ensures that blood is always pushed forward; it does not flow backwards because of the blood behind it. Instead, arteries must deal with increases in pressure when blood flow increases; as a result, arteries have walls that expand and contract in response to the pressure of the flowing blood.