Many claim that arm strength is an overrated aspect of quarterbacking and in comparison to some of the other characteristics, such as pocket presence and accuracy, that is true. However, it's still vital to have a strong arm because it has an impact on the design of the offense.
If a quarterback has a weak arm, the offensive coordinator has to eliminate some of the passing concepts, consequently limiting the offense. When concepts are thrown out and the passing offense is more based off of horizontal routes, it can be an issue moving the ball against defenses that like to sit and jump routes.
In contrast, a strong-armed passer is able to keep defensive backs on their heels by pressuring them vertically with the deep ball. The receiver who once was simply running vertical routes to clear out space for underneath receivers becomes a threat down the field and the defense is stretched horizontally and vertically.
In some cases, arm strength is the difference between winning and losing.