Involving functional managers in program development is the most essential element of an information security program, because they are responsible for ensuring that the information security policies, standards, and procedures are implemented and enforced within their respective business units. They also provide input and feedback on the information security requirements, risks, and controls that affect their operations and objectives.
Reference =
CISM Review Manual, 16th Edition, ISACA, 2020, p. 37: "Functional managers are responsible for ensuring that the information security policies, standards, and procedures are implemented and enforced within their respective business units." CISM Review Manual, 16th Edition, ISACA, 2020, p. 38: "Functional managers should be involved in the development of the information security program to provide input and feedback on the information security requirements, risks, and controls that affect their operations and objectives."