I have been wondering about these for a while now, too. When I moved here I even walked all the way around one, like a jack ass looking for the slot to throw my mail into, clearly with no success. Than finally last week, I saw the one by the front of my building open with our mail lady digging through it for more mail. I think it's basically a USPS locker to store mail to be delivered into, so their bundles don't get to full. BORING. Bring on the conspiracies Gothamist!