Because they suffer from chronic pain. Because insurance companies won't pay for less addictive ways of managing pain. Because some people don't have an extra $500 or more to pay for pain management out of pocket. Seriously. The NY Times recently ran a story about how insurance companies prefer opiates to other, less addictive, more expensive forms of pain management (e.g., Butrans patches). And this accords with family members' experiences - especially those who live in states without legal marijuana.