It's not just cancer. There are over a thousand known medical reasons between smoking and cannabis oils. I use it for my anxiety, glaucoma and insomnia as well as for recreational reasons. It also helps people with stomach issues, people with MS which you may want to YouTube. Its quite amazing!!! Helps with muscle aches, arthritis backpains in both forms. Believe it or not weed is classified as a depressant but it helps with depression if you consume the sativa strain. Cancer is the most common because that's where we find the most uses. It fights and beats cancer from stages 1- 4 (3 and 4 is when they are usually permanently hospitalized), helps recovering patients with pains and discomforts, helps them eat food and keep it down, alternative to chemotherapy which 1 in 12 times can kill the patient.
Must I really continue? You might be saying well why does a healthy person need to smoke then?? Smoking weed reduces risk of Alzheimers, epilepsy, stroke, and the forming of any type of cancer or tumor. I'm sure there's more but I can only remember so much. If you really want some serious information watch The Union on Netflix. I actually got my mother who was 64 at the time to change her stand point on weed. BTW I am of Korean decent and if you know old school Korean adults they are stubborn as hell in what they believe!!! She fought me growing up about it until I started doing my research and showing proof and evidence that what she was taught growing up was actually false or half truths.