Wing
The fact that it is called a Winglet instead of a wing tip, is instructive. Everything about aviation is expensive, and margins are tight in construction as well as at the Gate. Even the selection of a new word costs money. So though it is attached at the end of the wing, it isn't invested with the same purpose, exactly. Is it useful? Does Bill Boeing spin in his grave at the cost sheet? Of course they work. Different aircraft, different wings, different Winglets. They save Fuel because they convert some useless Drag into Lift, producing a well considered cost benefit.