For multi-lane networks with lots of traffic, sometimes you may want to let trains switch lanes. If one lane backs up, trains can switch to another free lane. However, the lane-switching train might cause a traffic jam behind it in the new lane! With Signals, you can make trains wait for an opening in the 2nd lane before merging.
In the above picture, train A is stopped due to a jam further ahead. Train B decides to go around it by switching lanes. Train C comes and makes B have to wait to merge. This allows B to not wait in the jam, but also not cause a jam in the 2nd lane.
This junction is very useful before large Railway stations.