From a medical marijuana perspective it wouldn't change much for states where it is still illegal. It will make things easier for people who are prescribed it in states where it is legal, and hopefully for places that produce or sell marijuana that are currently locked out of banking and payment systems. This would also allow Medicare to at least consider covering it in those states, but they wouldn't necessarily have to. Medicare coverage decisions are made by the center for Medicare and Medicaid services, we'll have to see after this change goes through what they determine. They do also already cover FDA approved medications based on cannibinoid ingredients like marinol or epidiolex which are pharmaceutical preparations of delta 9 thc and cannibidiol respectively (these are already available in every state since they are fda approved). Private insurance also will make their own determinations about whether they will cover it or not, but with this change there is a chance they could, whereas before there was no possible way. Medicaid coverage is mostly determined by each individual state.

The only way this would over ride state law and allow medical marijuana into a state that doesn't have legal marijuana would be if somehow the marijuana plant itself got an FDA approval, but that is very unlikely for a lot of reasons, foremost that the marijuana plant has a large mix of many different drugs with many differences in amounts and ratios of those drugs from strain to strain, plant to plant, different parts of the plant, or even the same plant at different times in its life. It's not like, heroin, or fentanyl, or cocaine which are specific chemicals. You could never really say "marijuana plants in general" have a specific indication for a specific disease, it would need to be much more specific in terms of what is actually being given, and only that would have the evidence and therefore the FDA approval. Like take epidiolex/cannibidiol for instance, a single chemical, 25 mg/kg/day was found effective as an add on therapy to another primary therapy for reduction in seizure frequency in children with Lennox gestaut syndrome and dravet syndrome. That's the specific indication and dosage that the FDA agrees is effective based on the evidence. Lots of other reasons too you'd never see an FDA approval for "all marijuana plants in general," but the unpredictable mix of tons of different drugs across many many strains of marijuana plants and variability between the plants itself is enough to make this a practical impossibility. It's definitely contributed a few medications that have roles in certain diseases though, like many other plants before it.

In short, you'll still need to convince individual states to legalize it or make medical marijuana laws if you want an actual marijuana plant or plant preparation prescribed to you. Medicare, Medicaid, and private insurance coverage could all be different (and even different by insurance company), but there's at least a chance it could give coverage now, whereas it was impossible before. This also makes marijuana research easier and helps reduce any federal criminal penalties.