Why are westbound transatlantic routes located hundreds of km away from eastbound routes?
The reason this is done is due to the winds aloft. The Jet Stream is a powerful current of air that blows in a west to east direction. Airplanes crossing the Atlantic from west to east take advantage of the jet stream to get there faster and save fuel, so a course is chosen to stay in the stream as much as possible. Airplanes going from east to west will be slowed by the jet stream, using more fuel and taking longer to get across, so a course is chosen to avoid the strongest winds. 

The difference can be very significant, I've been on flights from NYC to London which take 6h 30m, and ones from London to NYC that take almost 9 hours.