It can be both. Higher voltage allows longer small wires, and/or more power on the same wires. Depends on what the car needs. If it is just a few lights on the back of the car you are looking for smaller/cheaper wires. However if you are looking to put something power hungry in back (work trucks have a lot of needs around this) the higher voltage allows the same wires to deliver more power.
It’s not primarily about delivering more power, is it? I thought the point of higher voltage is that for a given power the wires can be smaller.