Speculation suggests that the Man Cannon is at your base or near a spawn point. Lets say its a gigantic map, and all the vehicles are gone. A man cannon can toss you in the middle, where the action is. It's also said that you can attack other players in mid-air, even if they are on the ground.
File:Mancannoneditin8.jpg
The Only Man Cannon Image
File:Mancannoninactionedithj8.jpg
Possibly a Man Cannon being used