Trade shows are an important cornerstone of both the American and world economy. They're one place where B2B companies can build relationships and sell to each other. For other firms that sell goods and services to consumers, trade shows foster an environment where they can build their brands and reputations.
Of course, a perk for guests and attendees at trade shows is the free stuff. Promotional products weren't necessarily invented with trade shows in mind, but certainly fit the environment perfectly.