The reason to use cold ingredients for flaky pie crust is that as the crust cooks, the heat from the oven explodes solid fat particles. These miniature explosions of solid butter and shortening release steam. In turn, the steam creates individual crisp layers of crust.    If you were to use soft or melted butter, on the other hand, a lot of that moisture has already escaped from the butter before cooking. Instead of crisp, light layers, your pie crust is more likely to be oily and soggy. For this reason, it’s important to use ice water and to avoid warming the dough too much by handling it or over mixing it.