The question as to the psychology of trade fairs is somewhat misleading. One could also just as well ask: what else besides psychology plays a role when it comes to trade fairs? Without psychology, or better, the psychological element, not much would happen at these events!
From a psychological perspective, there are two central issues at trade fairs: coming together and interacting. These two contradictory qualities per se have to be explained.
Coming together can be understood as the desire of all people to meet with, exchange with, and come into closer contact like-minded people. Related to trade fairs, this means that people want, for example, to see what all there is (or what is new) in their field. People seek out other people who seem to have similarities to them, correspondences, who complement their own preferences and inclinations. They feel good, feel inspired, widen their horizons, make friends and acquaintances. Trade fairs offer exhibitors and visitors alike (in many cases one and the same person!) manifold opportunities and possibilities to unfold in this respect.
Physical and sensual contact with others is especially valuable in the age of the so-called New Media. It is not only that the new kinds of media cannot replace trade fairs: they are what make trade fairs so important and indispensable today. Trade fairs benefit from the deficits or our non-sensual media much more than people think!