The better (and size can play a part here) your forwards are the more you will win the ruck, the more momentum you'll have, the more room (and time) the halves will have to run, pass, kick, plan, organise. If this (and hopefuĺly it does) translates into tries (or even penalty goals) then you get more possession. With more possession, momentum and confidence, comes less pressure; therefor increasing the liklihood of more discipline, fewer penalties conceded, fewer mistakes overall. It's very difficult to be disciplined, hang on to the ball, have a good kicking game and make sound decisions when you are constantly on the back foot and under continued pressùre.