What mystery shopping is, and who it is for
Mystery shopping is a simulated purchase visit: someone unknown at the location walks in, works through a scenario agreed with you and fills in an evaluation form immediately afterwards. The point is not to catch anyone out, but to show you what a first-time customer sees.
A sales report tells you how much was sold. A mystery shopping visit tells you why: whether anyone was at the shelf, whether the product recommended was yours or a competitor's, whether the promotion advertised at the entrance was still valid at the till.
Advance Service Advertising runs these evaluations for networks with several locations, where the difference between stores only becomes visible if someone walks all of them against the same criteria, in the same period.