In terms of helping your admissions chances, an on-campus interview can definitely help because it shows your level of interest in the school. If you have reason to think that a particular school might doubt your interest in the program, a campus visit will go a long way towards helping your cause. Schools know that these trips are time-consuming and expensive, which is exactly why they believe that an on-campus interview is a signal that you're truly interested. If you're not interested enough to take the time and spend the money to visit the school, then why should they be interested in you?
But, keep in mind that many people get into schools each year by only interviewing with off-campus alumni. I got into Kellogg after an off-campus interveiw, but I had also separately visited the school, and I made sure to let my interviewer know, and I also mentioned my visit in my essays.
The bottom line is that you don't have to do it, but it will give you at least somewhat of an advantage, particularly when it comes to showing your enthusiasm for the school.
Scott
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Co-Author, Your MBA Game Plan