Many of the pre-game and in-game strategic decisions in baseball
revolve around a fundamental fact: in general, right-handed batters tend
to be more successful against left-handed pitchers and, to an even
greater degree, left-handed batters tend to be more successful against
right-handed pitchers.
A manager with several left-handed batters in the regular lineup who
knows the team will be facing a left-handed starting pitcher may respond
by starting one or more of the right-handed backups on the team's
roster. During the late innings of a game, as relief pitchers and pinch
hitters are brought in, the opposing managers will often go back and
forth trying to create favorable matchups with their substitutions: the
manager of the fielding team trying to arrange same-handed
pitcher-batter matchups, the manager of the batting team trying to
arrange opposite-handed matchups. With a team that has the lead in the
late innings, a manager may remove a starting position player—especially
one whose turn at bat is not likely to come up again—for a more
skillful fielder.