The reason it works is that cannabis doesn't need anything at all to help it take root. The hormones for making roots are already present in the cutting.
It is believed that adding the hormone increases the likelyhood or speed of the cutting taking root.
Cannabis is genetically engineered to colonise in a number of ways. If a cutting is expected to fall from a plant in the wild and manage to find grounds for root then you pampering it with 100% humidity and decent light should be enough on it's own... and strangely enough it is.