We go back to 2003 for our story. Major League Baseball owners are meeting in Scottsdale, Arizona trying to rectify a public relations embarrassment at the 2002 all-star game. They decided on January 16, 2003 the best course of action was tweaking the all-star game. It would become more than an exhibition. Starting with the 2003 contest, the league that wins the all-star game will have home field advantage for the World Series.
The move was precipitated by Commissioner Bud Selig being forced to call the previous years game, being played in his hometown of Milwaukee, in the 11th inning with it tied 7-7 because both teams ran out of pitchers.
The thinking now is teams will be playing to win, so their league has home-field-advantage in the next World Series, not just to get everyone in the game. Teams will be urged to save pitchers and other position players for the eventuality of the game going into extra innings.
Fifteen of the previous seventeen World Series champions (before the 2003 rule change) had home-field advantage. The two leagues had been alternating home field advantage since the World Series began in 1903.
The story from January 16, 2003 lated about a dozen years. As of December 2016, the owners changed the home-field-advantage rule again. Starting with the 2017 post-season, home field advantage for the World Series does not go to the league that wins the All-star game. It goes to the World Series team with the best regular season record.