State registration is separate from federal and SRO registration. The Series 63 covers the Uniform Securities Act model: registration of agents and firms, prohibited practices, unethical sales conduct, and the remedies available to state administrators.
It exists because blue-sky-laws survive alongside federal law. A representative fully qualified with finra still cannot call a customer in a state where they are not registered.
The exam is brief and conduct-focused, but the underlying rules are not trivial: state administrators bring their own enforcement actions, and a state bar or fine appears on the public record just as a federal one does.
Related: blue-sky-laws, series-7, series-66, registered-representative, finra-brokercheck