Why give freebies?

We have all seen freebies at trade shows and we have all probably seen the person that goes from booth to booth collecting as much of it as possible. There is always someone that only goes to get the free stuff, but then again there are also people that go there to learn about new products or services. These people may be genuinely interested in what you have to say. Even if they are, there must be an element of what is in it for them (we call them WIFM - pronounced Whiffem