There are some problems that formulas just can't crack. One of these is solving questions that need trial-and-error guesswork.
In most Excel spreadsheets, you begin with a set of data and use formulas to analyze that data and calculate some conclusions. When you create plans and projections, however, you sometimes invert this process and start with the conclusions. You may begin with a profit projection and calculate "backward" to find out the sales you need to make. Or, you may calculate the rate an investment needs to increase in value in order to meet a set target.