Cabin Filter Question

I used the info I found here to find and change my cabin filters in my '06 Titan Crew cab LE. I didn't feel like I was getting enough airflow from the a/c so after pulling the filters, which weren't too dirty, I reinstalled the filter door and then got practically no airflow from the vents. I removed the filter door and found a square piece of foam about 1/4 inch thick that was blocking the ductwork. After removing, the a/c works like a dream. Any idea on where the sheet of foam came from or what it's for?