What's the mitral valve's purpose?

Backflow: The mv prevents blood from backing up into the lungs during left ventricular contraction.

The heart is a pump: Moving blood by squeezing. The valves keep blood moving in the proper direction. The mitral valve prevents blood from moving backwards from the left ventricular to left atrium. Also the way the leaflets are positioned it directs blood into the lv in the most efficient direction so it can exit thru the aortic valve with the least amount of effort. That's 1 reason repair is better than replacement.