An interview is an opportunity for an applicant to express an interest in attending the school although they certainly do not make or break a decision. They are also a chance for an applicant to learn about the college. Interviews are also more an indicator of what an admission result might be rather than a determinant of admission.
If you decide to request an interview or are offered one, you should prepare yourself with both questions to ask as well as responses. It is important to do some preparatory research on the institution.
The alumni interviews at highly selective colleges tend to reconfirm everything in the student’s application. Honestly, though, almost every interview report is positive so it tends to not have the same influence as the other pieces of the application. And, because most colleges can’t guarantee that every student will get an interview, they can’t put too much emphasis on them. I see the power of the interview report dwindling even though colleges will never admit this. If a student is applying to small, liberal arts colleges, the interview tends to matter a little more as it is usually conducted by an admissions staff member.