In an article on Investopedia, some examples of mystery shopping and how it can benefit the end-user, the customer, are explained:
A mystery shopper can check that an establishment has “brand integration”—otherwise known as providing a product or service that is consistent with the company’s image. If signage and advertising suggests one thing, but the goods or service delivery is quite another, a mystery shopper can help point that out.
Mystery shopping programs that evaluable customer services can ultimately save the customers of that business time and money, by not having to return items or leave a store mid-transaction. This, in turn, saves the company money when customers aren’t lost.
Keeping up health, safety
Mystery shoppers can fill the gaps between health and safety inspections by making sure stores and restaurants are keeping up with maintenance between the visits of inspectors. The reports of a mystery shopper can clue management in on possible trouble areas before inspectors are due to return.