Take the case of a restaurant where customers expect to receive good food and to be served by an attentive wait staff. Everything has to come together for each customer in order to meet his or her expectations. This is no easy feat.
Probably everyone has experienced an "almost good" meal, where one small aspect of the meal didn't go quite right. Perhaps the food was excellent but too long in coming, or maybe you had to ask for the check three times before you finally got it.
Whatever the reason, it is clear that people expect both the product and the service to meet their standards. If either fails, the entire sales event is a failure.