An actuary is someone who does actuarial work. Most people join the profession right out of college. They have no piece of paper other than their degree. They are still actuaries.
A credentialed actuary is someone who has gone through the entire educational system, which are a series of exams that are typically taken while working as an actuary. Credentials and the practice rights that come with them permit an actuary to do exactly one new thing. Only a credentialed actuary can opine on the adequacy of an insurance company's loss reserves (to make sure that the company has set aside enough money for all of the claims it will eventually have to pay).
Not many actuaries issue formal statements of actuarial opinion. All of the other work that we do can be legally done by anyone. There is nothing stopping an insurance company from hiring a large number of statisticians or whatever to do the day-to-day work, only bringing in a credentialed actuary to do the reserve opinion once a year. Companies generally don't do this because actuaries are more than just statisticians; they're business professionals with very deep domain knowledge. Credentialed actuaries command a premium in the marketplace not because of any "regulatory moats", but because the specialized education itself is very valuable. If that ever stops being the case then nothing is stopping the market from correcting itself.
Why does it matter that actuaries are certified? It's a professional role which requires dealing with some pretty heavy concepts. Employers presumably want to be sure that candidates meet some level of capabilities.