The answer is quite simple: whenever you photograph a person, product or property and try to sell the image certain legal implications arise.
For instance, is the person agreeable to you selling a photo of them? Would they complain if they saw their image used on a billboard? What about advertising a product that they don’t believe in? Do they even know you took the photo in the first place? Does the owner of the property know you will be earning money off their property? Do they consent to that?
Clearly, having release forms protects you legally.
But remember, microstock agencies are selling the image license to clients on your behalf. Thus they could bear some of the legal consequences if the model decides to sue. Can you imagine the microstock agencies opening themselves up to that kind of litigation on the millions of licenses they sell each year? Enter the model & property release requirement. It protects you and the agencies you will be licensing through.