I'm hoping to be at a booth at a job fair for my company in a few weeks (it's at my old college, alum connect).
I can already tell you that these are far more important for small companies vs. big ones. Say you want to work in insurance, for example. Liberty Mutual likely gets 100+ resumes a day, but a smaller one might get much less, and they may only have 1 recruiter on staff looking to hire 1-3 positions at a time. Since that recruiters' higher up is almost always C-level management (in a company of around 50 people, it would make sense for C-level to be engaged in hiring), it's usually someone that can get you noticed if they like what you have to offer.
Word of advice, be sure to research the companies that are there. Get a list of around 5-10 (or more if you want) and have a useful elevator pitch ready for each.