Marijuana can be used as a medicine and it is used in that way by many people. It has been known to be effective against pain and useful when other drugs are not, or the side effects are preferable over the side effects of other kinds of medicine. It has both a recreational and a medicinal purpose.
Why is this even a question? Anyone who does any research should know the medicinal value of the marijuana plant. Sure, people use it recreationally, an that is fine. This does not mean that people suffering from Cancer, Aids, Pain, Seizures, ADHD, Alzheimer's, Dementia, and many more diseases can't use Marijuana for treating these ailments.
Marijuana does have some health benefits but people use that too much. People just like to use marijuana recreationally and do not care about its health benefits and only enjoy getting the high. People will use any and all arguments for marijuana if it benefits them. It is not a medicine.