Why are croissants brushed twice with egg glaze?
In the baked goods where I double-brush, the first layer of eggwash is applied  before a long (cold) proofing. When proofing already formed buns, you don’t want the layer of foil or whatever you are using too tightly on top, both to ensure there’s enough room for expansion and to keep the top „pretty“. The wash will moisten the top of the buns and prevent (or at least reduce) the formation of a skin, which later may interfere with oven spring. It should also keep the crust soft. The second application right before baking is for more shine and extra humidity on the outside, again allowing for maximum oven spring and keeping the crust soft. 

I can’t see why the double-brushing would be specific for croissants, in my experience it’s more about proofing and a soft crust.