What 2 jobs do the valves of the heart do?

Both AV valves, the bicuspid and tricuspid, (separating the ventricles from the atria) prevent blood from flowing back into the atria when the ventricles contract. Both the semilunar valves (separating the arteries from the ventricles) prevent blood from flowing back from the arteries once it has been pumped out of he ventricles, and thus, out of the heart. The difference is that the AV valves are contracted when they are CLOSED, but the semilunar valves are contracted when OPEN. Otherwise, I can't think of another major job besides preventing backflow.