Short Story: As cannabis grows, the cannabinoids develop and change. They continue to synthesize after harvest and are alive and changing. Most people benefit from curing and decarboxylating their weed.
Longer Story: Cannabis produces cannabinoids. Initially there are 2 acids that form all of the cannabinoids in marijuana. In the beginning of its life cycle, these synthesize into CBGA. It has been documented that CBGA turns into CBG, THCA, CBCA or CBDA, and probably others. As life continues other cannabinoids are formed. THCA turns into CBNA and THC and other cannabinoids are formed or lost. During late flowering and harvest time, the acid (ones with an A at the end, or a carboxyl group) morph into what people call “bioavailble” cannabinoids. This means the body more readily uses them.
All of this occurs in the trichomes, or small appendages, of the plant.
One scientist states the acid forms are too large to pass the blood brain barrier. I have no idea how large or small a molecule needs to be to pass this barrier. but it is confirmed the acids lose a carboxyl group and therefore are smaller after this loss. It is also studied that the cannabinoid receptors (CB1 and CB2) are not activated by acid forms. However, there is anecdotal evidence that both THCA and CBDA are useful for some children. It has been shown cannabinoids do interact with different systems of the body. So if you want to try and use the “raw” form of the cannabinoids go for it. Do recognize that they will still change over time and the synthesis will continue. If you make cannabis oil without decarboxylating, it is still alive and will continue to cure over time.