Why should I hire you from the outside when I could promote someone from within?

TRAPS:  This question isn’t as aggressive as it sounds.  It represents the interviewer’s own dilemma over this common problem.  He’s probably leaning toward you already and for reassurance, wants to hear what you have to say on the matter.

BEST ANSWER:  Help him see the qualifications that only you can offer.

Example:  “In general, I think it’s a good policy to hire from within – to look outside probably means you’re not completely comfortable choosing someone from inside.

“Naturally, you want this department to be as strong as it possibly can be, so you want the strongest candidate.  I feel that I can fill that bill because…(then recap your strongest qualifications that match up with his greatest needs).”