Here's a hypothetical:

Trans Atlantic route where market supports a 757. Not enough pax to justify a 767-300.

Problem: sometimes headwinds mean a fuel diversion westbound.

Potential solution: someone crunches the numbers, and says the curb-feelers will save X amount of pounds of gas, cutting number of tech-stops to drop to Y.

Wingtip mods are expensive. But so are unplanned diversions, where fuel is higher priced, connections are missed, sometimes United has to put people up in a hotel, or the crew times out.

Just a theory.