Home field advantage
From BR Bullpen
The home field advantage refers to the edge which the team hosting a baseball game has over its opponent. While home field advantage exists in every sport because the host team usually benefits from better knowledge of the field on which the game is played, and will have a favorable crowd cheering it on, in baseball, the rules give the home team an additional benefit. That benefit is the possibility of batting last, which means knowing precisely how many runs it must score to win or tie the game, allowing the manager to adjust his strategy accordingly, i.e. playing to score one run or trying for a "big inning".
The practice of the home team batting last was not formally entrenched in the rules until the early years of the 20th Century, when it became mandatory. Before that, the home team could chose to bat first, and would sometimes do so to gain a tactical advantage (for example, by trying to rattle an opposing pitcher by forcing him to pitch in front of a hostile crowd before he had had a chance to adapt). Most managers realized however that the tactical edge of batting last in the final inning was much more precious than any short-term gain from batting first. In Major League Baseball, another rule that advantages the home team, in interleague play, is that the use or not of the designated hitter is dependent on the practice of the home team. In an American League park, the DH will be used, while in National League parks, it will not. This is to the advantage of the AL teams, whose roster is constructed with the use of a designated hitter in mind, while National League teams must use a player who is usually a pinch hitter or a defensive substitute in the role, not a "pure hitter"; the reverse is not so true, as there is little difference in the hitting ability of AL versus NL pitchers.
While in all games, the average winning percentage of any team is .500, its winning percentage goes up to .550 at home, and down to .450 on the road. That advantage has been fairly consistent throughout all eras of baseball, although some teams, especially those who play in a particularly atypical home ballpark, may have a bigger edge that endures over the years.
When a game is played in a neutral park, the home team is decided beforehand. There are some cases in which a visiting team is considered the home team, for example when a game that has been rained out has to be moved to the visiting team's ballpark in order to be replayed before the end of the season; in those cases, the "home" team bats last, even though it is not playing in its own ballpark.
During the regular season, each team plays an equal number of home and road games (although rainouts may intervene). In the postseason, the team with the better record has home field advantage in a majority of games; there are tiebreakers in case the two teams have the same record. In the World Series, home field advantage alternated between the National League and American League until the 2003 World Series, when it began to be awarded to the league which had won that season's All-Star Game.