Grew,
Winglets reduce the (apparent) induced drag by producing a small amount of thrust. They do this by reacting with the wing tip vortex flow, such that their total reaction is angled slightly forwards.
Because their angle is fixed they can be most efficient in only one, fairly narrow part of the speed range. So to get the best from them they are tuned to the cruise, where the aircraft spends most of its time.
I'm surprised to hear that they do much at all during the take-off run, but then designers contiue to improve things.
Today's conventional wisdom soon becomes tomorrows old wives tales (I hope that's not being too sexist)