The primary reason for creating a business case when proposing an information security project is to establish the value of the project in relation to the business objectives and to justify the investment required. A business case should demonstrate how the project aligns with the organization's strategy, goals, and mission, and how it supports the business processes and functions. A business case should also include the expected benefits, costs, risks, and alternatives of the project, and provide a clear rationale for choosing the preferred option.
References = CISM Review Manual, 16th Edition eBook1, Chapter 1: Information Security Governance, Section: Information Security Strategy, Subsection: Business Case Development, Page 33.