Why would a financial manager use the overall cost of capital for investment decisions when the specific decision under consideration may be funded...

Why would a financial manager use the overall cost of capital for investment decisions when the specific decision under consideration may be funded by only one source of capital, (e.g., debt or equity)? no minimum word count. Please add source if possible

Why would a financial manager use the overall cost of capital for investment decisions when the specific decision under considerationmay be funded by only one source of capital, (e.g., debt or...