Over the last two decades, baseball has become a highly specialized sport.
Starting pitchers throw five or six innings before handing balls off to middle relievers, all of whom have a very specific role. One is the long man, and a couple are there to get teams through the fifth and sixth innings.
You have a "LOOGY," a left-handed specialist who only exists to get out tough left-handed hitters. You have a seventh-inning man, a set-up man in the eighth, and then your closer in the ninth.
And as Baltimore Orioles manager Buck Showalter showed in the American League Wild Card game, there are times you're not supposed to bring in your closer, who is usually your best relief pitcher.