Assuming that
DrummerAlly purchased the lunch that they referred to after getting their FP, that's
PirateFrank's case in point. Money was given to Disney by a guest making a purchase instead of waiting in a queue.
Whether FP was created as a convenience for the guest and any benefits to Disney are incidental, or it was created as a benefit to Disney and any conveniences for the guest are incidental, is of no consequence. The fact of the matter is, the "brilliance" of the system is that it (at least appears to) serves
both.
And anyone who thinks that these large scale decisions are made by saints who
only have the guest's convenience in mind are simply
wrong. It's not
convenient to have to dodge merchandise and other guest congestion while exiting an attraction through a gift shop...