The answer is very simple : there is 0 benefit for a pure domestic itinerary, apart earning a few miles (and XP for a few weeks). If you have an international connection G3 (in Y) to AF/KL (in P or J) you can use the G3 priority counter at check-in. In SDU, for instance, there is a sign mentioning clearly AFKL P and J pax at the priority check-in counter. But this is not due to the status but to the class of travel on the international flight.