Most job descriptions have a long list of responsibilities, must-haves, and nice-to-haves. When you ask the interviewer a question such as this one, you ask them to narrow down the most important function in this role. Once the interviewer gives you the reply, you can target your answers to that critical function or need.

For instance, if you are applying to a marketing director position and the interviewer tells you that the most vital purpose is to ensure that all projects move along smoothly, you know that project management expertise is critical. You can continue your interview by focusing on your project management experience and PM-related education.