“Give and you shall receive” is a proven principle in life and business. Businesses have long seen the truth in this, receiving valuable, sales-generating appointments in return for giving useful promotional products to prospective customers.
Their success in landing sales meetings -- and increased sales -- is an often repeated example of the rule of reciprocity, one of the six principles of persuasion that Robert B. Cialdini, Regents’ Professor Emeritus of Psychology and Marketing at Arizona State University, covers in his book The Psychology of Persuasion, which has been on the New York Times Business Best Seller List.
Giving back in return for something given is a behavior ingrained from childhood in many cultures. From our upbringing we feel obligated to return favors. Effective sales people use this principal along with promotional merchandise to dramatically increase the number of appointments they are able to secure. 
Sending an introduction letter that includes a small but useful promotional product, not only introduces the sales person's company but greatly increases the chance that the prospective customer will return the "favor" by agreeing to a meeting. Of course this assumes the salesperson's company provides something the prospect actually uses or needs, but that's for a different discussion.
Cialdini cites several examples of this principle in action. For instance, the Disabled American Veterans, a nonprofit organization raising funds for veterans assistance, receives an 18 percent response rate to its mailed solicitations for donations. But when its fund-raising letters are accompanied by address labels personalized for each recipient of the plea, the response rate nearly doubles – to 35 percent.
The value of each set of address labels, the promotional merchandise they give away, is just 8 cents. The value to the fund-raising organization is exponentially greater.
The same is true when your company provides business prospects with promotional merchandise as part of an introduction. Depending on your business an appointment with a decision maker can be extremely valuable, just think about how difficult they are to get. The appointment is the first step to closing the sale; including a useful promotional product in your appointment generating process ensures that first step will be successful.