The human mind and brain is fascinating. It has a creative as well as a rational side. In marketing we have to appeal to both of these sides. Human beings are not just the cold and rational things traditionally assumed in textbook economic theory. Rather, the human brain operates on different levels, taking short cuts and so can easily get confused and influenced. There are plenty of tests which have now been done to prove that human beings can be nudged into making decisions (or perhaps the word should persuaded).
This all sound very psychological, but after all, marketing is just psychology. Promotional merchandise is a nudging tool. Use promotional merchandise to help nudge the right people in the right direction - this makes for effective marketing.