@travisejones @alatiera It's likely in place so future software that requires a birthday from the system can retrieve it.
For example, some mature video game with adult elements could ask systemd for the birthday before letting the user play the game.
AFAIK, there are no hooks in place that verify that the supplied birth date matches their birth certificate, government issued ID, etc. So there's nothing stopping you from lying about your age to get around the video game rating.