Why do we use the overall cost of capital for investment decisions even when only one source

Question:

Why do we use the overall for investment decisions even when only one source of capital will be used (e.g., debt)?

Cost Of Capital
Cost of capital refers to the opportunity cost of making a specific investment . Cost of capital (COC) is the rate of return that a firm must earn on its project investments to maintain its market value and attract funds. COC is the required rate of...