Mark Kleiman illustrates the need for it:
It appears that one local grower developed a strain sold under the “Purple Urkle” label. It was widely held, by producers and consumers alike, to be truly righteous weed, and it flew off the shelves.
Then the fashion for chemical testing came in. Purple Urkle tested at a mere 7% THC – perhaps twice the THC content of what was called “marijuana” when I was in college, but well below the 12-18% that current products claim (more accurately in some cases than in others). Result: even the consumers who had already experienced and enjoyed Purple Urkle, and had been asking for it by name, wouldn’t touch it. They were so used to the idea that quality is defined by THC content that they didn’t want to smoke what they now “knew” to be weak weed. So the brand more or less died.
In principle, a legal cannabis market could improve consumer satisfaction and safety by delivering products of known chemical composition. But if the heavy users who dominate the market in terms of volume have a prejudice in favor of maximum THC content, the practical outcome could fall well short of the promise.
McArdle lists other areas in which transparency hasn’t helped the market.