My summer job consisted of dressing in a Hawaiian T-shirt, listening to the same IZ cd everyday, and handing out smoothie samples. I was working for a coffee and smoothie shop my mom's friend owned that was located in an H.E.B. A large part of the job was handing out samples when the store was slow since it was relatively new and a lot of H.E.B shoppers were unaware it existed. Of course the owner had a plan in mind by handing out the samples. She didn't do it just to give me an extra thing to do (even though it felt like it at times),but instead she hoped that by giving people something for free they would be inclined to want to be nice back and purchase a drink.
The idea that people feel like they should return acts of kindness is know as the norm of reciprocity(Gouldner, 1960).In other words, my boss hoped she would appeal to a social norm of treating others how they treat you. She was also good about giving people discounts when I was training if I took to long to make a drink. This way when people took our free samples they felt obligated to buy a drink, and if they received a discount they felt obligated to come back and buy one full price the next time.

Gouldner, A. W. (1960). The norm of reciprocity: A preliminary statement. American Sociological Review, 25, 161-178