The point of the Sessions action is to clarify the constitutuionality of the federal law.
The Congress passed a law forbidding marijuana. The States have passed laws regulating the sale of marijuana . There is conflict.
Thus, some states attorney will bring a law suit that will be decided by SCOTUS. That is why we have three branches of government.
It's been several decades since SCOTUS and constitutuionality had much to do with each other. Justice Thomas seems to be the only consistent federalist on the Court.