1. It is reportedly a better pain killer in some cases than opioids so it keeps people off of the prescription opioids
2. It is nowhere near as addictive (possibly not addictive at all)
3. When people are already criminals (because they smoke weed) I think they are more likely to say "screw it" and try a harder drug. In the vein of, "I'm already doing something I can go to jail for, what's one more thing?" Legalizing keeps people out of the criminal class (increases barrier of entry to it)
I'm actually for legalizing all drugs like several other posters but I don't think society is ready for that.
I got in a argument with a close relative (a retired police officer) and another close relative (a doctor) about this on Easter. There arguments were entirely based on emotion and lot so logical fallacies. And understandably, they have both seen the darkest sides of what drugs can do. But criminalizing doesn't help those people.