In my experience with Ph.D. interviews, it is primarily to get a sense of how you interact with the PIs, the level of maturity in your responses, and clarity in your future goals and ambitions is essential. Other than that, as others mentioned, you need to demonstrate a strong link between your interests and what the program/professor has to offer. Ph.D. is a very serious commitment, and you need to demonstrate you are taking this decision after due consideration. I would recommend having good questions prepared to ask before the interview as well as some follow-ups to showcase you know the program well and have specific and thoughtful questions.