Familiar surroundings for you. The types of fields represented is limited because participating employers must know their hiring needs one year in advance. These agencies are helpful when relocating to a new area where you're unfamiliar with employers. They're a good use of time because you can often meet several employers in a short period of time. You can present yourself in person.
Many fairs target specific fields.
You're often competing for employers' attention with many participants. The fair may not be related to a field of interest to you. Networking puts you in contact with people who will remember you and may recommend you for upcoming positions that you wouldn't have known about. This method requires initiative and assertiveness on your part. You aren't interviewing for actual positions, so you have to have patience.