Power
You wield a lot of power over businesses by working as a mystery shopper.
You have the power to make service changes for the better. If you've always wanted to tell on that rude waiter or nasty bank teller, or complain about the cold, greasy burgers that take too long to make in a fast-food joint—this is your chance.
Some people may get fired based on your reports. You have the power to do that. You also have the power of praise. If someone's doing a stellar job and you let her boss know, maybe she'll get a promotion.
There are even "Reward Shops"—one of the few times you will be asked to reveal yourself after the shop is over so you can reward the employee with an "Attaboy" or "Attagirl" good job certificate. These kinds of shops are rare, but they're fun. You get to feel like the guy on Candid Camera. Who doesn't like being caught doing something good and getting a reward?