The belt runs against the tensioner lever/roller and keeps your belt firmly on the cam gears and crank. It makes sure the no teeth are missed and your engine is running with the correct timing. Keeping tension on your timing belt is imperative for keeping everything turning properly. If some slop develops in your belt because of a poor tensioner lever/roller, the timing set can skip a tooth and cause catastrophic damage and potentially ruin your engine. This replacement will keep everything tight.
