From http://m.mlb.com/glossary/transactions/postseason-roster-rules-eligibility - Teams carry extra players throughout the postseason in the event of injuries, and those players, as well as players on the injured list, can be in the dugout during games, within reason. So Astudillo travels in case Garver or Castro go down with an injury, to be available the next game. He could probably even be a bullpen catcher or warm-up catcher if needed.