I consider an interview as the last chance for the school to check for 'red flags' specifically psychological instabitilies. They are observing how you behave in person (there are some nutcases that look fantastic on paper but horrible in person, like weird and mentally deranged) and your maturity level. so.....apart from that it's also an opportunity to explain any weaknesses in your file.
That's the way I think of the interviews. Don't BS, be honest, explain bad grades/low scores etc, and don't act weird (or at least learn how to act normal with people if you do have social inadequecies, when you're a doctor they will be call eccentricities)
When I got rejected after the interview, I brushed it up to limited size, b/c I got accepted at another school. But that's the way I rationalize interviewing process.