Triple-A teams' main purpose is to prepare players for the Major Leagues. ESPN wrote in 2010:
Winning is nice, but secondary. It's much more important for a young prospect like outfielder Xavier Paul to get regular at-bats against lefties, or work on dropping down sacrifice bunts with a runner on first, than it is to take three of four from the Portland Beavers.
Both young players and veterans play for Triple-A teams:
There are the young prospects speeding through the organization on the fastest treadmill, the guys who used to be young prospects who are in danger of topping out in Triple-A, the 30-somethings trying to get back to the majors after an injury or a rough patch, and the guys just playing a few more seasons because someone still wants them and they still want to.
Players at this level who are on the 40-man roster of a major league team can be invited to come up to the major league club once the major league roster expands on September 1 (though teams will usually wait until their affiliates' playoff runs are over, should they qualify). For teams in contention for the post-season, these players create the flexibility needed to rest regular starters in late regular season games. For those not in contention, recalling such players gives them an opportunity to evaluate their future players under game conditions.
Teams at this level are divided into three leagues: the International League, the Pacific Coast League, and the MLB-independent Mexican League. The Mexican League fields teams throughout Mexico. The International League traditionally fielded teams in the Northeastern United States, and now fields teams in the Midwest and South as well. The Pacific Coast League originally fielded teams on the West Coast, but now fields teams throughout the western part of the United States. For much of the 20th century, the American Association, which consisted of teams in the Midwestern United States, was also at this level, but it disbanded in 1997 and its teams were divided among the IL and PCL. Each of the 30 Major League Baseball teams has an affiliation with one Triple-A team in the United States. However, Mexican Triple-A teams are not included in the organized farm team system.
The Triple-A All-Star Game is a single game held between the two affiliated Triple-A leagues. Each league fields a team composed of the top stars in their respective leagues as voted on by fans, the media, and club field managers and general managers.