There are two kinds of model searches: Searches put on by agencies, and searches put on by scouting services.
Agencies hold searches in order to find new talent and to get leads on people who might be interested in buying classes.
These searches are usually held in the same town as the agency, but may sometimes go into the surrounding areas to recruit new talent. Some of the bigger agencies, such as Ford, will go worldwide looking for the next big thing.
Often, in smaller searches, in smaller centers, there will be a nominal registration fee and international scouts will be brought in to pick winners.
Scouting services, on the other hand, will go through large and small cities, filling the airwaves with their offers to introduce you the top modelling scouts and agents.
Generally they will book a meeting room in a hotel and open the seminar to the public.
A small presentation will be made telling the basics of modelling, and how the only way to get discovered is to connect with people in the industry.
Next, some attendees (usually about half) will be invited to a bigger gathering where international scouts will be present.
I've attended several conventions, and some are better than others.
If you are planning to attend one of these events make sure you understand what you are getting into before you sign up. Have a long look through this site to determine what agencies are looking for. When the scouting service invites you to their event it is up to you, not the organizers, to make sure you are ready.
Are models searches legitimate? Most are. Just be careful before you spend a lot of money, and don't fall victim to any organization that forces you to decide on-the-spot.