I have seen videos of this happening in real life, and my conclusion leads me to believe it happens for one, or a combination of the following reasons.
In older aircraft, without the more modern and advanced autothrottles, the PF sets the approximate takeoff thrust, and then the PNF may tweak the power to the appropriate takeoff thrust setting.
It may also arise from older aircraft and prop's with adjustable throttle friction. Having two sets of hands prevents the throttle slipping back.
I'm sure someone would be able to clarify these points.