Question:

Why must you add ice cold water to the dough when making a pie crust?

Answer:

Your goal is to use just enough to create a flour/water matrix that'll hold its shape, but not enough to potentially make the crust tough. In addition, using ice water helps the fat remain cold and solid; and the colder the fat when you put the pie into the oven, the greater the chance for flakiness.

Source: www.kingarthurflour.com/guides/pie-crust/