If you want to act in movies or on television in Hollywood, you must be a member of the Screen Actors Guild. That’s why the accession of a “SAG” card is such a momentous achievement for actors. It means they’re Hollywood legal as it were.
So why does acting professionally require union membership? The answer is very simple: SAG wants to prevent those yearning to be stars from flooding the market, and subsequently bidding starting wages down to zero.