What effect does focusing a Pilot played as an enhancement have? (The Outrider unit text is what prompted this)

The same as focusing any enhancement card, really - once it has a focus token, you cannot focus it for any other reason. (Such as drawing a resource from a Pilot with the Black Squadron trait while Guarding the Wing is in play.)

Yup, and some pilots grant special abilities such as those attached to outrider or Mauler Mithel etc.