That's a good answer, butI don't think it's "official", is it?
Besides, it begs the question, why doesn't Battle 7 have a configuration which doesn't need to be transformed to swivel the cannon?
Thanks. It does make sense, but like ewilen pointed out, they could very well make a turreting cannon instead of transformation. Transformation takes alot of precious time which leaves the ship vulnerable for those precious few minutes.
Well, a conventional turret only gets you 360 in one plane. You still have to move the ship if the enemy isn't oriented in the same plane as you(I assume a gun that big would be restricted to one axis of motion). And it'd likely be oriented to hit the least threatening vehicles, the ones coming in along the front and sides. The top and bottom, your largest surfaces, are undefended.
In humanoid form, your gun defends one of your most vulnerable sides.
Then there's the psychological factor.
The ship likely "OH MY GOD IT'S HUGE I'M GONNA DIE!" effect on people on the wrong end of the gun.
...
And most importantly, it looks cool.