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.
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.
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.