The first thing you credit an NFL team's success to is either the quarterback or running back.
Like the counseling exercise, tell me what pops into your head when I say something.
Colts? Peyton Manning
Patriots? Tom Brady
Vikings? Adrian Peterson
Chargers? LaDainian Tomlinson
Cowboys? Tony Romo
But there is an element to NFL success that originates before the quarterback or running back—the offensive line.
The offensive line affects everything.
This is what happens if a team has a good offensive line (hypothetically).
The quarterback never gets pressured. He always has time to find open receivers. Also he never gets sacked, so he can maximize his passing efficiency because he gets the most chances possible to connect with a receiver on every pass play.
Since the quarterback never gets pressured, he can get the ball to the receivers much more easily, especially if his receivers can run routes well and create separation from their defenders. The fact that the passing game is working opens up the running game.
The running back always has room to run. He can always find a hole or at least pick up a couple yards on every play because the offensive line can hold off the opposing defensive line.
The rest of the opposing defense starts further from the line of scrimmage for fear of getting burned on a deep pass, giving the back plenty of room to run before he encounters even one or two defenders.
The defense keeps running in circles trying to keep up with the offense's play-calling. This makes them out-think themselves. The offense can then play a ball-control game, maximizing the amount of time the opposing defense is on the field, which tires out the defense much more quickly.
With the defense tired and unfocused, the offense can do whatever it wants.
Then when the roles reverse and the offensive team's defense comes in and vice-versa, the defense has a good lead to work with. It can just go out and concentrate on doing whatever it needs to do to stop the opposing offense instead of focusing on one thing.
It can take more risks, which keeps them on their game and prevents them from relaxing and allowing the other team to get back in the game.
The defense is also rested as much as possible, making their play that much more effective. They have far more of a spring in their step than the offense. In addition, the opposing offense stays off the field longer, disrupting their rhythm badly and causing them to get rusty as the game goes on.
Soon the opposing offense goes three-and-out, and the process starts all over again. The team with the good offensive line wins.
Now this is what happens to a team with a bad offensive line.
The offensive line can't protect the quarterback. The opposing defense sacks him multiple times, banging him up and severely disrupting his timing with receivers. That gets the whole offense out of sync.
Because the passing game never got off the ground, the entire defense can cheat toward the line of scrimmage as it cracks down on the run. The running attack gets shut down.
The entire offense collapses in on itself, and before they know it, the other team has the ball with excellent field position. The field position issue gets worse depending on the quality of the team's punter.
The team with the bad offensive line will hang around with the other team, but not for long.
Eventually the offense's ineffectiveness forces the defense to stay on the field for too long, exhausting them only half of the way through the game.
In the second half the opposing offense can do whatever it wants to. It stays on the field for minutes on end, and the defense is vainly trying to keep up.
Eventually the opposing team starts widening the gap more and more, and in time the game becomes unwinnable.
Face it: NFL teams must have high-quality offensive lines to win. It's that simple.