I believe these are ventilation gratings or something similar. The one in my room vents to our attic, and there are a couple that vent straight to the outside through the walls.
Perhaps they're there to make sure that even with modern tight-fitting double-glazing, there's still plenty of oxygen getting into the room, and to provide an exit path for carbon dioxide and moist air.
Various types I've seen are floor vents (possibly combined with underfloor central heating), slim metal vents in the walls, larger metal-covered ones, small ones with closable flaps built into double glazing (if you have these, keep them open at night - they stop condensation building up, especially in bedrooms) and the ones fitted to our 1967 house, which are like painted perforated bricks. | 
