Although you do not need a resume for the interview, preparing one can be helpful in other ways. It can help you organize experiences that you will eventually put on your application. I gave a copy of my resume to my letter of recommendation writers to give them extra ammunition for their letter.
I don't think it's a waste of time because you essentially are doing exactly what you will be doing when you fill out the actual application. Having made a resume prior to filling out my application made it easier to complete it. I think it helped.