There is a ton of info over two episodes, like they even visit one of the Chinese laboratories under the guise of being a potential customer, really worth going through the whole thing.

On the specific topic, it's cheap and differentiates the product. Someone who buys it will get a different (better?) high than they can kind anywhere else, and they're more likely to come back for more. It can also be added to substandard product to make it seem "better". It's not a long lasting effect either, so people will want more sooner.