Wingtip modifications will save Alaska Airlines $20 million annually

[FBaggins]

3. The boundary between a lifting surface and the air around it...

... necessarily creates drag from turbulence. By transitioning instead to a surface that isn't lifting (thus creating less turbulence), you reduce drag. Here's an example of the original move to upswept wingtips:



The same kind of thing is done on propeller tips to reduce cavitation, aircraft props and on wind turbine tips to cut down on losses due to drag.