The pundits will tell you it's a passing league. That you can only win with a quarterback that can take over the game and make eye-opening throws on third down and long. That the running back position has been devalued and can be acquired from the scrap heap basket. But what they didn't tell you was that the running game is about to become more important than ever before because of how it changes the passing game and takes advantage of the smaller defenders.
It's been a few decades since the passing game became popular and it's been a few years since it has exploded into the NFL limelight. Quarterbacks are passing the ball at an alarming rate from spread formations that were introduced in the 1950's by Dutch Meyer and up until recently were labeled "gimmicks".
Offenses are using the entire width of the field to stretch defenses out horizontally and force man coverage -- even when it's not intended -- against crossing routes that span from sideline to sideline. When receiving threats aren't running across the field, they're catching short passes into the middle of it that are coming so quickly that the defense doesn't have time to align properly nor attain the fundamental form of tackling to separate the receiver from the ball like they once did.
As a result, defenses have focused on becoming quicker and smaller in order to cover the spread receivers. The quicker and smaller a defender is, the less steps he is likely to waste getting to his destination and the pass catcher.
Also, these smaller defenders are being used in sub-packages against "11" (1 back, 1 tight end) personnel, which can still be an effective grouping to run from. The more smaller defensive backs that are on the field, the more success running games will have because of the mammoth pulling blockers will be able to engulf the defenders as witnessed by the New England Patriots running game the last two years.
Furthermore, defense’s have also become more accustomed to playing with an even coverage shell, meaning more 2-deep safeties and more bracket coverage.
The 49ers use the running game effectively to draw in a safety
In layman's terms, bracket coverage is double coverage of a receiver. When a defense is playing with 2-deep safeties, they are able to bracket or double a receiver underneath and over the top with a cornerback and safety. As a result of this, the running game becomes a factor once again.
It's vital that a team has a strong running game to lean on because it changes the way defenses play. Take for instance the Houston Texans; they are built on a zone running game that carries the West Coast Offense passing game.
The Houston Texans' running game attracts defenders into the box, namely the strong safety, and leaves less pass defenders for defense to cover with. Consequently, when the running game is rolling, the strong safety comes up to the box more frequently and the defense, in general, is more susceptible to the vertical passing game, which is a big reason why the Texans are so efficient passing the ball. The same holds for the San Francisco 49ers, who are also built on a running game -- power opposed to zone, however -- and are able to attack defenses vertically with the play action pass after successfully running the ball.
That's not to say that these two teams are the sole reason why an offense should run the ball more. They are arguably two of the best teams in the league, if not the best, but look at the effect on the defense that the running game has: the strong safety is coming down to the line of scrimmage, leaving only a single deep safety to cover.
If there's one thing that the single-high coverage shell can't do, it's bracket multiple wide receivers like the two-deep coverage shell can.
Regardless of how and with who it is done, running the ball more often and successfully will eliminate more of the 2-deep safety shells that defenses are looking to play and take advantage of the smaller sub-package defenders, resulting in a more productive and balanced passing game.