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. The interview will reveal how you respond to questions spontaneously. The way you answer questions and the content of your answers will assist in the process of image creation. These images will either confirm or contradict information in your file. Any live contact with the school will leave an impression.