Interviews for professional schools are the same as interviews for jobs. In a job interview your job is to market yourself to an employer. In an interview for a professional school your job is to market yourself to the school. A good question.
Interviews consume a great deal of time.
They are difficult to organize. Therefore, a school will conduct an interview because they think that a live interview will provide information that is not available from other parts of the application file. The application file is a careful and deliberate marketing initiative. Applicants have time to think about how to respond to the questions. Some applicants develop the file over a period of months.