They put method in your madness. A well-constructed valuation system can do exactly that. While the dollar values in your spreadsheet will only be as good as the forecast data behind them, the same can be said of more arbitrary valuation systems, and at least the spreadsheet takes care of the heavy lifting for you.
Sensitivity analyses are important for two reasons.
First, you may be unsure of the accuracy of some of your forecasted player stats. Say that you're not sure whether Carlos Febles will steal 20 or 40 bases this year. By altering his value in the spreadsheet and recalculating all the dollar values, you'll quickly and easily take the change into account. Doing so by hand would be tedious and not necessarily worth the effort. The second benefit is that you can quickly account for players who are traded out of the league or who suffer major injuries before Draft Day.