If the statute does not define the term "motor vehicles", then that term will have to be interpreted if questions arise in a court of law. A person driving a motorcycle might be pulled over and the police may try to fine him if his motorcycle is not registered with the DMV. If that individual argued to the court that a motorcycle is not a "motor vehicle," then the court would have to interpret the statute to determine what the legislature meant by "motor vehicle" and whether or not the motorcycle fell within that definition and was covered by the statute.