Shadow Shopper is a third party job board. So, they do not provide shops themselves. The shops posted on their site are posted by various MSCs. You can find the same shops posted on the individual company's job board. Some shoppers sign up for third party job boards like this in order to see more shops in one place. It would be up to you whether or not you think it's worth it for that reason.
I would note that the jobs advertised on third party job boards are not always the most up-to-date. Many companies post their jobs there once a month, but they do not take them down when they are filled. Not all jobs from a company are necessarily posted on these sites either. The individual company job boards will always have the most up-to-date information. However, it would give you an idea of what might be posted in your area by a company (even if it is not available right now).
Companies do perform recruitment through third party databases (e.g., calling or emailing those with Shadow Shopper, MSPA, or Mystery Shop Solutions accounts). However, this is usually after trying to fill a shop through their own databases first. So, if you are signed up with the company already, you will be contacted through your account with them before you are contacted on Shadow Shopper (or another third party site).