September 1 is among the most important dates on the MLB calendar. Always falling with only weeks remaining in the regular season, it allows teams to expand their big league rosters from 25 to 40.
All franchises call-up promising prospects whose minor league seasons have ended to determine whether they have a chance at making next year's "Top 25."
Unique to winning teams, however, is the activation of injured regulars. Those who have fallen out of contention are more inclined to shut down hurting players for September because frankly, they aren't playing for much. But the roster expansion is very convenient for the competitors who can bring proven players back into the fold while retaining the reserves as well.
There are merely 13 clubs with a chance to play into October this season (all others are 8+ games out of one of the eight spots), and those getting reinforcements certainly have an advantage. These are the notable additions since the month began.