Good question. It's up to the player how he control his tank, sometimes (s)he finds it more comfortable to drive the tank backwards in some cases. There is no limitation for that in this game. You can even pull off cool tricks and stunts with your tank.
Some hulls, such as dictator and, very unnoticeable, hornet, have the turret mount farther back then forward. By having your tank being backwards, you can fire off a shot quicker with less of your hull sticking out.
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