Certainly not to add flavor. The National Association of Writers of Pie Recipes (NAWPR) is not trying to see who can create the world’s sourest pie crust.
Vinegar can serve two useful roles in pie crusts – it promotes tenderness and can keep the crust from getting too brown.
with most pastries, you do not want a lot of gluten to form in your pie crust – those stringy, tough sheets of protein created when flour is mixed with water. Gluten makes the crust tough, but you want a tender pie crust – it is a badge of honor for a good baker.
Acids attack protein molecules and cut them down to size, minimizing the formation of long strands of gluten. Vinegar brings water to the dough, which is essential to the creation of the crust, but also a mighty, gluten-slaying acid. Lemon juice, buttermilk, and sour cream often show up in pastry recipes, as well, in whole or in part for the acid they bring.
Acid added to a crust also inhibits browning. This is especially useful for the crust of a long-baking pie, such as pecan pie, where the crust might brown too much.
In general, we believe, recipe writers add vinegar for its tenderizing effect, rather than the color-control issue. But to be honest with you, we don’t see a lot of pie crust recipes that call for the addition of vinegar.