Why did major league teams evolve to designate their best relief pitcher as the closer, who enters the game only in save situations? It began in the 1950s, when relief pitchers started to pitch more innings than starting pitchers:
Teams carried fewer relief pitchers before the 1980s. Generally, they had just one or two “firemen”, who are ace relief pitchers, and a few mop up guys for low-leverage situations. They almost always entered the game before the 9th inning with men on base. Bruce Sutter was the first relief pitcher to start more than 20% of his career games in the 9th inning. John Franco was the first pitcher to hit 50% in 1987, and Lee Smith broke 75% in 1994. As far as baseball history goes, the modern closer is a very new thing.
Up until the late 1960s, relief pitchers posted worse ERAs than starting pitchers. But that changed as, and relief pitchers now post lower ERAs than starting pitchers:
Why are ERAs lower?…