Final answer:
The expanded stage direction suggests that the rebels' cause is strongly supported and signifies a disciplined and unified military force, which is a vital element in the context of the battle narrative.
Step-by-step explanation:
The expanded stage direction describing "many soldiers, all marching in perfect unison" primarily contributes to the scene by demonstrating that the rebels' cause is widely supported and that they are a formidable and organized force. This unified movement of troops signifies discipline and a collective determination which can significantly intimidate the opposing army. It implies that the rebel army operates as one entity, likely enhancing their combat effectiveness and morale.
Perfect unity in soldiers' movements often indicates thorough training and a high level of discipline, essential qualities of a capable military force. Therefore, the stage direction not only illustrates the visual impact of the soldiers' coordination but also suggests the underlying strength and support for the rebels' cause, which are critical elements in the narrative of a battle. It can also create a stark contrast with the potentially disorganized or less disciplined forces they face, though it does not necessarily confirm the combat outcome.