Hi Peter!
Please be open and honest. If for example, you want to get the opportunity to have a deep look into the daily work of a consultant, want to teg to know the team dynamics on a project and simply learn about how it "feels" to be part of this profession/firm before doing a 180 degree career switch, then just say it! This is 100% okay and very understandable! A reasonable firm will also want to make sure that you know what you sign up for, instead of taking the risk of having to part ways after a couple of months. In case of a successful internship, this risk is more or less eliminated.
Cheers, Sidi