First of all, yes you are correct – you could make some money on the side to supplement your income. You can even make a part time job of being a shopper.
Besides the money; however, think of the company you are working for when you evaluate their workers. Companies rely on good and great customer service to keep their business running. Too many times a bad employee makes a customer leave and never return. Your job when you mystery shop is to let the manger know, through your report, exactly how you were treated. Good or bad the manger would use your report to reward the better employee.
Moreover, after some time you would see what characterizes better than regular customer service. This is a useful skill and you could carry the knowledge to your own job and make yourself a better employee. In time your newly found skills can even give you insight to a better outlook at work. Hopefully this gives you a raise or some type of advancement.
Your other abilities, such as accurate note taking and observation skills in a clinical manner, will also open more doors for you in your professional life. Entering reports in a timely manner is very useful to employers as well as strong writing skills. As you know all mystery shoppers do these types of things on a regular basis.
So, instead of seeing yourself as just a “mystery shopper”or as a part time gig for some money think of all the new found skills you have now learned to lead a better life.