Why do managers often use several relief pitchers?

The strategy: Create favorable matchups

Kenley Jansen 2015
Right-handed batters hit just .151 with a .459 OPS against Dodgers closer Kenley Jansen in 2015. Sean M. Haffey/Getty Images

One of the biggest questions that fans new to baseball ask is why there is a parade of relief pitchers in the late innings of a game, especially in close games.

It’s not because the relief pitcher – who might have thrown to only a batter or two – is tired. It’s because the manager is going by the book.

It's all about the breaking ball

It’s more difficult for a left-handed hitter to hit against a left-handed pitcher, and it’s harder for a right-handed hitter to hit against a right-handed pitcher.

Those are two of the most basic baseball truths.

The reason is physics. A curveball or slider thrown by a left-handed pitcher will break down and away from a left-handed hitter, and vice-versa. This makes it harder to hit.

The theory is proved by statistics. For example, take the 2015 National League MVP -- Nationals outfielder Bryce Harper. In his MVP season, the left-handed-hitting Harper predictably mashed against both lefties and righties, but he still did markedly better against righties. Harper hit .335 with a 1.160 OPS, 35 homers and 74 RBI in 367 at-bats vs. right-handers. Against lefties, he hit .318 with a .986 OPS, seven homers and 25 RBI in 154 at-bats.

In Harper's career, the difference is much more noticeable. He's batted .301 with a .954 OPS vs. right-handers. He has a whopping 79 homers in 1,267 at-bats. Against lefties, though, he's hit only .259 with a .784 OPS. Harper has averaged one homer every 31.3 at-bats against lefties.

His home-run rate is almost twice as good vs. righties -- one per 16 at-bats entering 2016.

Conversely, left-handed pitchers fare much better against left-handed hitters. Take Blue Jays reliever Brett Cecil, a seven-year veteran who made the switch from ineffective starter to strong reliever. The lefty was good against both left- and right-handed batters in 2015, but lefties really struggled against Cecil.

Left-handed batters hit .195 with a .539 OPS, one homer and 33 strikeouts in 77 at-bats. Righties hit .198 with a .576 OPS, three homers and 37 Ks in 121 at-bats.

The top-ranked closer in the game entering 2016, the Dodgers' Kenley Jansen, was very good against both lefties and righties in 2015. But it was right-handed hitters who really didn't fare well against the righty flame-thrower, hitting .151 with a .459 OPS.

Counter measures

Managers try to counter this by using pinch hitters. Teams like to keep a balanced bench so they can use a right-handed hitter to pinch-hit against a left-handed pitcher, or a left-handed hitter to pinch-hit against a right-handed pitcher.

Sometimes, the hitter called on to pinch-hit might not even face the pitcher. For example, let’s say a stud lefty is on the mound and a left-handed batter who traditionally struggles vs is due up. Because it would be a poor matchup, the hitter's manager would call for a right-handed player to pinch-hit. But after he’s announced, the pitcher's manager could decide to call a right-handed pitcher from the bullpen. To counter that, the hitting team's manager could call for a left-handed pinch-hitter.

Thankfully for all involved, the substituting couldn’t go on infinitely.

There are only so many pitchers in the bullpen and so many hitters on the bench.

Edited by Kevin Kleps on April 3, 2016.