Why do recipes for pastry dough tell you to refrigerate it for a half hour before using?

--Ashlan

There are three reasons for chilling pastry dough before proceeding.  First, and most important, is that during the cutting in of the fat, usually butter or lard,  and mixing of the dough, the fat has had a chance to soften.  If you were to proceed directly from mixing to rolling, you would press the softened fat into the flour granules.  What you want to do is have the fat roll out into flakes between layers of the dough.  Refrigerating the dough cools down the fat and allows it to harden back up.  Once it is hardened, it will roll out into the desired flakes.

During mixing, you are trying to evenly incorporate the water completely into the dough while not mixing so much as to form a lot of glutens.  Glutens are formed by the combination of proteins in flour with water.  They are encouraged and strengthened by mixing, so stirring in the water creates glutens.  Some recipes attempt to minimize this by adding a bit of vinegar or other acid, which inhibits gluten formation.

All of this brings us to the second and third reasons for letting the dough rest.  Resting allows any glutens that were formed to relax, creating a more tender pastry.  At the same time, during resting the water in the dough will spread out, making for more even hydration and therefore a more consistent result.

You should consider the half hour to be a minimum resting period.  Pastry dough can be made up a day or more in advance without loss of quality, provided it is refrigerated until you are ready to use it.