That's how ship cannons worked too, at the time. Powerful but heavy to move, slow to reload, not very accurate.... having more of them was the only way to have sustained firepower.

But Leonardo also left a lot of these sketches that look less like actual projects and more like the superhero fantasies of an extremely gifted six years old. "And look, this shit has cannons... Cannons EVERYWHERE! Bam! Kapow!". I guess it's what happens when you're so great at drawing that even the doodles you do when bored look like masterpieces.

They used muzzle loaded cannons at his time, so it makes sense. It's not only for direction, but for faster rate of fire.

Why shoot at one target when you can shoot at 30? (as long as they're encircling the machine in a nicely spaced out, orderly fashion...)