I listened to an interview Ned Yost gave on 810 WHB, and he talked about bullpen strategy. The Kansas City Royals changed the view of bullpens during their World Series runs. Lock down offenses after five innings with a group of closer type relievers. It was a cheaper way to get quality innings than the cost of hiring starters.
This strategy worked so well, the rest of the league hopped on it quick. The cost of quality relievers has since jumped dramatically. This was especially true for the players bouncing back or returning from injury that the Royals found cheap back then.