why don't Boeing leave wingtips vertical like on A350 or 737? A feet of vertical winglet replaces 3 feet of horizonal wingtip. So with 3 feet high winglets Boeing cold have just avoided the need for folding.
vertical winglets are ways to get more lift:drag out of a wing that's constrained by some width limit (airport gate spacing) or an existing wing design that you don't want to redo. they don't generate lift. for a clean sheet design you can do better with a "scimitar" wingtip that actually provides some lift and might weigh the same as a winglet. see 787. also the navy p8 which has the most efficient wing of any existing 737, and doesn't have vertical winglets (because it doesn't typically park at a gate they could go wider with these little triangular extensions)