I've done this for years, and it does help us guesstimate whether we should make changes. We make substantial money thru our investments, which are not captured by normal withholding. By making a projection, it helps us adjust our withholding, or occasionally we've made estimated tax payments. We also used this method to estimate the impact of various retirement dates.
Won't help for 2017 to 2018 though - too many changes to the tax code!
