a faster speed your car is on the road a shorter amount of time. If we are close to having a traffic jam, say during rush hour, by having people just before the jam drive faster there are less cars on the road and you can delay the jam. By driving faster you can get more throughput from the same number of lanes unless you also have to increase the distance between cars creating a situation where the fewer cars on the road are a bigger hindrance than the faster speed improves through put.

If you then get stuck in thick traffic, the stop and go is by far a higher cost than the increased speed.