I've have read many times that pie dough is meant to be held sparingly to avoid creating gluten, but kneading the pie dough does exactly that. So, what is the motivation?
Recipes usually specify lightly kneading pie dough to help it hold together better (especially when fitting it into the pie dish) and to orienting the gluten and fat into flaky layers instead of crumbly crumbs. Over kneading blends the fat and flour into a mortar/paste that takes longer to cook through and is tough and cardboard-like.
Handel the least amount possible. You mix pie and Quiche dough, puck it then let it (the gluten developed from mixing) rest in the fridge for 30 minutes, then roll it out. Otherwise you are developing the gluten (like bread or pasta) and working all the cold fat out in turn you are unable to trap the cold fat between the flour and wont have the flaky layers needed for a Clark crust