Make sense when your low. So the question "In a sportstar, why is there a need to keep a hand on the throttle during takeoff". The reason is to make sure that you don't loose power while your low and slow.
In the sporty the throttle has these notches, like a thread. When you apply full throttle, an let go of the button the throttle can be sitting on top of a notch which means during takeoff the throttle can pop-in to a lower notch causing a small loss of power. A small loss of power during the ground run or when low can be rather serious! I have only had this happen once, maybe twice.
Generally, I keep my hand on the throttle atleast until I have completed my crosswind turn (maybe 600, 700 feet). If I am doing circuits then I leave it on until I have reached circuit height. (whats the point of taking the hand away for such a short period, I guess).
My explanation isn't the only reason I am sure but it is particular to the sportstars kind of throttle.
Steven.