Just joined the forums and saw your question.

A W9 is used by an individual (or business) to disclose their taxpayer ID (social security number for a person, or EIN number for a business) to a company. The reason that you must disclose your taxpayer ID to mystery shopping companies is that if you make more $600 or more, the company is required to report your earnings to the IRS, which requires your taxpayer ID.

It is important to note, however, that not all mystery shopping companies require you to fill out a W9. Some just ask you for your taxpayer id to be entered into a field in your profile, and some do not ask for your taxpayer ID until you make $600.

I hope that answers your question. If you have any additional questions, feel free to reply here.