According to the CISM Review Manual, 15th Edition, the primary objective of an information security management system (ISMS) is to align the information security strategy with the business strategy and ensure that information security objectives are consistent with the business objectives1. This helps the organization to achieve its goals and protect its information assets from threats and risks.
References = 1: CISM Review Manual, 15th Edition, Chapter 1: Information Security Governance, page 11.