I have no idea and don't know the difference but I could say the same thing of the camera they used to take my passport photo or my state ID.
But more to the point the reason for doing it is there, for better or worse — want a passport? Have a picture taken. In this case I am (at least for now) comparing opting out or not. You can still take your flight without them taking that additional picture. That of course may not stay true.
https://travel.state.gov/content/travel/en/passports/how-app...