The way the defense helps the offense is two-fold
1) it can take pressure off them. The offensive players perform better when they know they don't have to put 40 points on the board to win.
2) If you are only good enough to score on every four possessions, the defense can increase the number of opportunities to possess the ball by forcing a punt or getting a turnover.
Now that is simple and everyone knows that of course, but it is fact- and it does work the other way, as well. No unit on the team performs in a vacuum. With a better defense, Fitzpatrick will put up better numbers, but once we get to that stage, he will have to be replaced with an upgrade or we won't take the next step