The purpose of using 10 deg of flap for takeoff is that it reduces the takeoff run by producing more lift at slower speeds. The first stage of flap on the C150 (10deg) is all lift and very little drag, the next stage is more drag then lift which is why it is used for landing (20deg) and the 3rd stage(30deg) is all drag and is used for short field landings.
So the first stage is a small deflection and produces more lift and very little drag therefore reducing the takeoff run compared to no flap.
Any clearer?
buckle up
Nick
<<edit: Stands back to wait for "Incoming!!">>