When is it necessary to put foil over a pie's crust?
Generally speaking, when the length of the cooking time required to cook the filling correctly is such that the crust would burn if it wasn't covered. 

It is usually more necessary when you have blind-baked the crust, but I must say I've cooked a lot of pies in my time and never covered the crust because I'm always careful to par-bake the crust just enough to seal it and no more.