When you roast coffee, there's a bunch of chemistry going on, a bunch of browning reactions, and almost all of those produce carbon dioxide as a byproduct.
It allows for perfect color and browning of your pie while, at the same time, giving you enough time at a lower temperature to really bake those apples and allow them to become tender.
But it has all this really awesome burnished bits from during the browning period, and this is sort of like a flavor bomb of like beautiful, toasty vanilla-y notes.