Although the NEIS are voluntary standards, they provide details that clarify what constitutes good workmanship in various tasks involved in electrical installations.
NEC 110.12 and the other NEC rules addressing electrical workmanship can be subjective for Code enforcement officials — especially if they have limited experience in the electrical field. The NEIS help describe what the installation should look like and how it should perform to meet this minimum requirement in the Code.
These voluntary standards have been adopted for use in some jurisdictions. If the NEIS are referenced in project plans and specifications, they usually become requirements and are enforceable by inspectors.