My guess is, as with most things in aviation, it all started before the days of automation. I would want to believe that before autothrottles came into being, the PF would have his hand on the control column after rotation, while the PNF and/or FE would have his hand of the power levers/thrust levers/throttles and monitering engine parameters. Thinking back to flight school, I remember being taught to keep a hand on the throttles during initial climb out to prevent them from being retarded due vibration.