Last September, 22 players were designated for assignment as teams cleared spots on their 40-man rosters. One reason to open a 40-man spot is to add a September call-up when the active roster expands.
For example, Joel Sherman of the New York Post says the Yankees intend to promote Jesus Montero, but not Manny Banuelos or Dellin Betances. Montero will have to be added to the Yankees' 40-man roster, which is currently full. A spot can be created by designating someone for assignment, or transferring a player from the 15-day DL to the 60-day.
MLBTR's 40-man roster count post indicates that 11 teams currently have at least one open spot on the 40-man. Montero, Martin Perez, and Devin Mesoraco are a few top prospects who would have to be added to 40-man rosters, notes MLB.com's Jonathan Mayo, and the Yankees, Rangers, and Reds are all currently full.