According to maturity models (like CMMI), a "mature" process is one that is quantitatively managed. You cannot improve what you do not measure. While policies and training are necessary, they only show that a program exists. Metrics provide the evidence that the program is actually achieving its objectives and reducing risk. A mature program uses data to drive decision-making. Measuring metrics allows the Information Security Manager to demonstrate "value delivery" to the board and adjust the strategy based on real-world performance.