They allow basics to have controlled pricing, and increased availability, while preventing abuse of the system (ie, alcohol purchases).
Having dispensaries (or dispensary-specific items in a general store) where you can use your food stamp money would leverage a working system we have now.
You would still have the problem that it wouldn't actually provide food for everyone or some people would end up losing the money because they're frugal.
So while food stamps (universal or need based) may not be a perfect solution (selling food stamps for drug money is not unheard of), it is still better, because it makes misappropriating the help provided more difficult.
But it doesn't matter because far more people will use it for good things like feeding and clothing their families.
On balance, that's better than what we do now, which is that the addicts still abuse substances, but many who are not addicts struggle to provide for their families.