Your investing now but need to know if the cash flow will be lucrative or result in a loss. Using a NPV formula will help you determine what the value of that cash flow is worth today. A positive NPV means you will make money while a negative one means you won't.
It is important to remember that this calculation doesn't take into account actual risk. One could make the assumption that there is no risk. Cash flow could stop, rise, decrease, etc. depending on circumstance and the market. Most "big time" investors use multiple formulas, calculations and actual "know how" to make things happen.