There's a hole in the door

On the front door to my apartment there is a mysterious hole located on the bottom right-hand side (when viewed from the exterior hallway). I have no idea what the hole is for or why it's covered with such an elaborate metal face-plate (which, when slid open, reveals an empty cavity not large enough for a letter but not too small for a bomb). Every door in my building has one. Any guesses would be welcomed.