I believe they're the current "cheat code" to the hiring process. If you get called up for an interview after talking to a company at a career fair, there's a good inclination that you have already passed or at least have a leg up on the "culture fit" part of the interview.

With hundreds to thousands of people spamming every job posting under the sun the second it is posted, just getting your credentials read by a human is a major barrier right now that career fairs help alleviate.