The session is non-evaluative. Come with an open mind and ready to take notes, the purpose is genuinely to try help you do your best in the interviews. McKinsey (like other firms) doesn't necessarily want to miss out on potential superstar candidates just because they had not heard of consulting/case interviews before, so this is them trying to "level the playing field" with those that will perform strongly just as a result of doing countless hours of prep.
Of course, If you have any burning questions about the interview specifically, it would also be the perfect opportunity to ask.
Enjoy the session and good luck with your interview!