Could be many reasons, one of them to hide components out of view - on a non tremor SD versions theres a metal piece which protrudes and can be easily removed, valence hides it. Also, it’s a trend nowadays to add valences and ford follows along. And of course it actually has to do with aerodynamics, not questioning the science. But 70 miles = 4-5 gallons win because of a larger air dam, i don’t think you believe that one either.