I was said that it is used because of different angles of attack on the different wings. Since the airplane flies always at some positive AOA or else it would descend (with symmetrical airfoil), therefor the rising wing would have a positive number plus something and the descending wing something minus the same amount (which is less then the rising wing) and the result- the rising wing produces more lift and drag.
What I don't understand is why Petec says it would be different in inverted flight, since I at least don't change the airplane's trim in inverted flight.