In stop and go traffic, one needs to use low gears to accelerate your vehicle. Usually, you would go over 3000rpm on first gear, 2500 on second gear then 2000 on third before you need to stop again. The distance travelled between traffic lghts is short and you would have consumed more fuel due to reaching higher RPMs.

When crusing on the highway, you can hit the sweet spot of 80kph * 2000rpm thus allowing you to cover more distance while not consuming as much fuel as in stop and go traffic.