To my best knowledge, that is the only test on iSSPrep that has a fee, all others are entirely free. Of course, as all other tests, it is completely optional.
It can be useful both to new shoppers, and those willing to engage in mystery shopping more actively and professionally.
It's there to give shoppers an introduction to (and an overview of) the fundamental concepts applicable to collecting business intelligence from the field, as it provides insights into the the Mystery Shopping concept, complements the knowledge gained in the iSS 101 test, and gives a larger points reward than any other test, ultimately equipping shoppers with the knowledge and shopper score increase that can be helpful in understanding more about the industry itself.
It was designed as a way for the people at iSecretShop that have been in the industry for over a decade and interacted with dozens of Mystery Shopping Providers, to share their knowledge and insights with shoppers that are really interested in fully understanding the entire concept, expanding their knowledge, and taking the "professional" approach to Mystery Shopping.
It is in no way a requirement for anything, nor promoted anywhere in particular, nor are the shoppers who take it favored over others - it's rather an optional "boost" to a shopper's knowledge, understanding of the industry itself, and their score, and can be handy for all shoppers that are eager to "dive into" the entire experience more and know more.
I hope this helps!