A good offensive line can help out all facets of an offense. Few quarterbacks can make do with multiple defenders in their face. Having capable pass blockers can not only keep your quarterback from getting pummeled, but it will allow for receivers to make bigger plays downfield.
The run game is also heavily dependent on great blocking. A good offensive line can make an average running back look like a Pro Bowler—it is quite easy to run when your line creates a truck-sized gap in the defense.
The hardest part, of course, is amassing a talented starting five.