Maybe 2.d) To prioritize lines. This use case happened to me:
I had a goods line sharing railway with a passengers line between two towns. I wanted to prioritize the passenger line so that passengers got a better service. I built sideways tracks and waypoints and signals for the goods train to park every now and then and give way to the passengers train. But, no signal in the main line. That way, the passenger train once reserves the track, it reserves the full track from origin to destination and achieves full speed. If a goods train is occupying the main track, it will release it as soon as the next sideways track is reached.