Why you go for an Interview?

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 have to be a Business major to go to Business Horizons, and you don’t 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 web site linked from the career / job fair list), and see what kinds of jobs each employer has.
To learn more about employers than you can learn from their web site. 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 web sites, and the like. Take advantage of opportunities to meet employers face-to-face.
Some fairs include follow-up interviewing as part of the fair, for a full or half day. Each fair’s web site should tell you if they do this.
Some of the employers who attend career fairs also participate in the On-Campus Interviewing Program. Meeting students in person at fairs gives them another way of screening candidates besides just the resume you submit for On-Campus Interviewing.
Regardless of the extent to which technology makes it easier and faster to share information between job seekers and employers, nothing replaces in-person contact for making an impression.
To be effective at a career fair, you need to be ready to make a good impression in person (just as you will be evaluating organizations by the way their representatives behave in person). To do this…