To make a good (or great) impression in person (especially important if your resume doesn't necessarily stand out from the crowd).
To see that the real world is not organized by major: you don't necessarily have to be a business major to go to Business Horizons, and you don't necessarily have to be an engineering major to go to Engineering Expo. You DO have to look at the list of employers attending in advance (see each fair's website linked from the career fair list), and see what kinds of jobs each employer has.
To learn more about employers than you can learn from their websites. You learn about the culture of an organization when you meet their people, and you can ask questions.
Much of the job search process — before you can even get an interview — for both you, the job seeker, and for the employer in trying to find good candidates, is not done in person. It involves employers screening resumes and cover letters, and you reading about employers and viewing their websites, and the like. Take advantage of opportunities to meet employers face-to-face.
Some fairs include follow-up interviewing as part of the fai