Pro Football Focus has a great article examining the importance of generating pressure on the opposing quarterback. When we talk about the pass rush, most of the focus tends to be on sack totals, but simply generating pressure can alter the effectiveness of opposing passing attacks in big ways.
When looking at passer ratings from last season you can easily see the drastic effect that pressure can have even when the quarterback is still able to get off a pass: Aaron Rodgers led 37 qualified quarterbacks last season with a 93.8 passer rating while under pressure; 27 of those signal callers had a passer rating higher than 93.8 on attempts coming from a clean pocket. The average NFL passer rating fell from 99.3 to just 64.6 last season when pressured.
When looking at interception rates we see a very similar trend; there were 219 interceptions thrown by these 37 quarterbacks on 12,100 pass attempts from a clean pocket, a rate of 1.8 percent. When pressure was applied that rate shot up to 3.1 percent as they combined to throw 153 interceptions on 4,942 attempts.
The main crux of the argument is the focus should not be exclusively on sacks when we evaluate the quality of a pass rush. The article singles out players like Brandon Graham and Chris Jones who have low sack totals but high pressure rates as quality pass rushers.
I have never seen a study that goes into such statistical depth, but it shows a truth of the NFL. Almost any quarterback in the league will look good when given a clean pocket. The average quarterback had a passer rating of almost 100 when the pocket is clean. In many ways what separates the successful pro quarterbacks from the ones who aren’t successful is the ability to deal and succeed with pressure.
When you are playing against a subpar quarterback, you can make him look like a star without generating pressure.
Things get trickier against the Aaron Rodgers, Matt Ryan, Russell Wilson, and Tom Brady types who still succeed when pressured, but their success rates at least go down when they are under duress. To at least have a chance of slowing them down, you have to rush the passer effectively.
On many levels this seems like common sense, but this article has detailed proof. I strongly recommend giving it a read.