It saves time - you could meet more than 50 potential clients in the same place in the space of two days.
It saves money - how much time and effort would it take to contact all of these specifiers and then visit them all at their office? It cuts back on your sales and marketing department's expenses.
It gives you the opportunity to find out about the needs and budget of potential clients, and to choose which key decision-makers you would like to meet.
It allows three suppliers per product category at most so there is limited competition.