It can help move traffic up to unblock exits further behind, which reduces congestion for those not actually travelling through the bottleneck. Once example I

It can help move traffic up to unblock exits further behind, which reduces congestion for those not actually travelling through the bottleneck. Once example I see often that is more length sensitive is dual left turn onto a highway onramp that merges to one lane before entering the highway. If people use both lanes equally all the way to the merge point you can fit more cars through each turn light cycle and then the merging process continues even when the light has changed. Often people merge early and leave room in one lane, which leaves some cars stuck on the other side of the traffic light who could have made it through.