The easy answer would be "Keeping the weak hand off the gun leaves it available for gestures or defensive action to protect the gun when shooting isn't necessary against the primary threat or in reaction to another, as yet unseen, threat that may appear from another angle."
The truth is that it was not, to the best of my recollection, scripted... it was just what Robin did intuitively. As for me, I teach to use a one handed ready position with the off hand prepared for other activities whenever a threat is within 2 arms reach or there are blind corners/bystanders within that distance.
-RJP