You can guarantee this question will be asked. The firm is trying to assess where else you are applying, where their firm fits in, and how much work you've done to prepare. You have 2 ways to handle this kind of question.

First, you can give them a few reasons - 3 reasons why you want to work for McKinsey, or 4 reasons why Deloitte is your first-choice firm. You need to explain each reason, and let them know at the beginning that you've thought about this question a lot - to reduce the risk of sounding totally memorized.

Second, you can tell a story. Stories are the best - they are believable, personal and very memorable. The story might sound like this: "I used to think investment banking was for me…and then I met this guy from McKinsey." Take insights from your networking conversations, and get specific.

No matter which option you choose here, make sure you address the question with enthusiasm - be excited about what the firm has to offer.