Hi, when I first had the gas engineer in to service my gas fire with back boiler I had no vent and was told that without one he would have to condemn it and shut down fire and boiler. Got vent fitted to outside wall and as TIA says got quite a draught coming through lounge so I put bit of foam inside to cut it down a bit. When the gas engineer came next time he told me to take it out or he would have shut it down. He said it was needed in case there was any build up of Carbon Monoxide in the room like if the
chimney got blocked or something and could not get out.
If it's a new build why not ask the builder the reason for it. He should be able to tell you. If you block it up it could invalidate your house insurance too so best be safe than sorry, especially with gas.