An example of this I was recently explained: There's a leg connecting Tokyo and Mexico City. The Tokyo flight goes direct to Mexico City however the Mexico Cit

An example of this I was recently explained: There's a leg connecting Tokyo and Mexico City. The Tokyo flight goes direct to Mexico City however the Mexico City flight stops in Hermasillo on the way to Tokyo. Due to the higher altitude of Mexico City, it takes more fuel to get off the ground and requires a pit stop in hermasillo to get all the way to Tokyo. Disclaimer I'm only partially sure that I'm remembering the whole story correctly.