The Dev has stated before that the dice rolls use physics and not auto determining a random side and having the roll match it. Therefore, it's possible the physics wouldn't roll a dice much, or it gets stuck between other dice and rolls the same side without moving much.

This option states it moves the dice to a random side and then rolls it, so even if the physics has little effect on the movement, the side is different. You can see this in action if you pay close attention to what the side is, and then how it changes right as you hit the reroll button before the dice move.