There are many differences between Federal and state laws. If some activity is legal in terms of Federal law, it might be illegal according to the state law. And if an activity is legal in terms of State law, it might be considered as illegal by the Federal law.
However, the first case mentioned here is viable, as this allows a generic administration. But in the second case, there will be issues between state and federal governments, which might result in confused administration – impacting the people.
For example, lets consider the marijuana policy between the federal government and the State government of California. Selling marijuana for medical purposes is legal in Californian state. But Federal government termed it as an illegal activity, even it id for medicinal purposes. The state has medical dispensaries, which sell marijuana legally. They even have permission to promote their product using even TV ads. But the federal agents started creating hassle by raiding these dispensaries and charging them against marijuana possessions.
The federal government is now considering putting a stop to this conflict permanently.