Geobeats, a creative network and video channel dedicated to exploring little known information, offers an interesting explanation as to the purpose of those tiny holes that drilled into the bottom of airplane windows. The reasons are simply balance and safety during altitude change.
That’s just the breather hole or bleed hole and it’s there for your own safety. When you reach higher altitudes the air pressure drops. Meanwhile in the airplane the cabin is pressurized to keep oxygen contained. The hole is only present in the middle window and helps keep it from breaking. It allows lower air pressure from outside on the outer pane and the higher cabin pressure on the inner pane to be held in balance.