The purpose of valves in the peripheral veins is to safeguard that the flow of blood in the veins is moving the proper direction, toward the heart. The flow is especially important in the legs where blood must follow a relatively long distance against gravity. Veins have valves which prevent backflow of blood which taken up from other parts of the body and is directed toward the heart.
This blood has low pressure, so valves help to provide pressure to make the blood move. The valves prevent blood from sinking down and settling into the legs, mainly due to gravity. A valve is there to make sure that blood doesn't move in the wrong direction.