From the Skin Cancer folks:
Today, systems for testing and determining UPF are similar around the world. In many countries, including the US, the ASTM International (formerly called the American Society for Testing and Materials) criteria for UPF assessment are used; UPF labels in the US often state that an item meets ASTM International standards.
Does all of this mean that everyone should specifically buy UPF-tested/UPF-labeled clothing, which most often carries a brand name? Is it so superior to everyday clothing that it is worth the extra expense and trouble to find? Not necessarily. Some items of clothing, such as denims and corduroys, are among the most sun-protective of all garments, UPF labels or not. However, a specially made high-UPF shirt, say, with long sleeves and a double layer of fabric at the shoulders — a high UV exposure area — might be constructed with a lightweight material that gives the wearer superior comfort and coolness as well as added sun safety. And the UPF label always adds a measure of certainty.
Knowing the rating is useful, but I suspect that manufacturers are using the concept for marketing at a higher price. A one-dollar cotton shirt with a tight weave will give the same protection as a $100 shirt with a name brand label.