Largely flexibility - you can build consists that contain multiple driver cars and still allow passengers to move between them, and then you can split them up a

Largely flexibility - you can build consists that contain multiple driver cars and still allow passengers to move between them, and then you can split them up and rearrange them without having to remove and reattach separate cars first. BART actually switched from angled front cars to flat front ones when they increased their fleet size, and converted a bunch of the angled ones (some were kept for effectively PR reasons). But the alternative is that you're losing space, since the maximum length of a car is determined by the track geometry and clearance of the system. Anything other than a flat fronted car is reducing volume available for passengers.