It began as a marketing move. In the 1970s, instead of serving in-flight meals, Southwest Airlines served peanuts. But they wanted customers to feel thrifty, not deprived, so they branded themselves as the “peanuts airline”—since you fly for peanuts, you get to eat peanuts. (Southwest was not the first airline to serve peanuts, but they were the first to serve only peanuts.) The snack became de rigueur after that.