It exists because the income-statement is built on accruals. Profit can be reported while cash drains into inventory and accounts-receivable; cash can pour in while losses mount. The cash flow statement is the referee.
The three sections answer three questions: did the business generate cash (operating-cash-flow), what did it spend on assets (investing-cash-flow), and who funded the difference (financing-cash-flow).
Example: Northwind Tools reports $78M of net income, $164M of operating cash flow, negative $92M of investing cash flow and negative $51M of financing, for a net cash increase of $21M.
Related: operating-cash-flow, investing-cash-flow, financing-cash-flow, free-cash-flow, income-statement