Proponents of marijuana legalization promote the benefits of its use. Research does demonstrate benefits from some of the ingredients in marijuana, but it appears that most of the benefits can be found in the non-psychoactive component known as CBD which is found in the hemp plant. It's important to know both marijuana and hemp come from the plant species Cannabis Sativa L.
"Cannabis is one of humanity’s oldest crops, with records of use dating to 6000 years before present. Possibly because of its early origins, and due to restrictions on scientific inquiry brought about by drug policy, the evolutionary and domestication history of Cannabis remains poorly understood. Drug types of Cannabis (marijuana), which contain high amounts of the psychoactive cannabinoid-tetrahydrocannabinol (THC), are used for medical purposes and as a recreational drug. Hemp types are grown for the production of seed and fibre, and contain low amounts of THC. Two species or gene pools (C. sativa and C. indica) are widely used in describing the pedigree or appearance of cultivated Cannabis plants." 
In my opinion, THC is the ingredient that marijuana users most covet because of the psychoactive effects it has, including "feeling high". Unfortunately, THC has many negative effects including short or long term psychosis in susceptible individuals. Why risk having psychotic episodes when one can derive so many medicinal benefit from CBD from hemp as explained in information below (see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3736954/ for more detailed information)