When a team has to play a doubleheader, they are allowed to add an extra player to the roster just to account for the extra strain that doubleheaders have on a team’s roster. This is particularly relevant for the Braves as today’s doubleheader against the Nationals is the beginning of a stretch where they play 22 games in 20 games without a day off.
Usually, teams add a pitcher just due to bullpen usage considerations. Running relievers out twice on the same day is a rough proposition, so having an extra arm is often very helpful.