And why, when a winglet is removed, does the airplane not roll uncontrollably if they are so effective?
The 747 wing droops when full of fuel and is substantially higher when low on fuel. Because of the sweepback, the winglets must be out of the airflow direction when either drooped or raised, and only streamlined when at mid fuel. There must be a drag penalty at the beginning of the flight and at the end, so are they really any use?
Or just look good.