The only OCU that you can actually carry while you use it. Most control units are bulky, heavy boxes that need to be set on a table or other flat surface to be used. They tie you down to one spot, and can't be quickly moved if necessary. What if someone starts shooting at you? You'd have to just leave the OCU and take cover, and your robot would be stranded.

And if the robot you want to control is small, what's the point of having a small, portable robot with an OCU that's bigger than it is? I mean, the robot can carry itself around, but you have to carry the OCU around.