Why use boiling water in a shoofly pie? I made a shoofly pie using the recipe from Joy of Cooking. That recipe instructed to mix 1 cup of boiling water into the mixture (of eggs and molasses) before pouring into the pie shell.

Why boiling water? What does that do for the pie differently from just using hot water?
If the recipe you are referring to is this one or something quite similar, it appears that it is in essence a variation of a custard pie, but the baking time is unusually short.  It is likely that the boiling water is to bring up the temperature of the mix quickly, to shorten the necessary oven time.