Thoughts on Joshua

I finished the book of Joshua this morning, continuing my goal to read through the Bible. In the third chapter, The Israelites followed the ark of the covenant into the Jordan River. The Jordan, being at flood stage at that time, stopped flowing so that the Israelites (including 40,000 soldiers armed for battle) were able to cross over to the opposite bank, close to the city of Jericho. Later, all these Israelite men were circumcised. They celebrated Passover and the manna stopped falling from Heaven. The Israelites were able to eat grapes, olives, the produce of the land. In chapter 10, the Israelite army, following Joshua's leadership, was involved in a great battle with the Amorites. On the day that his army claimed victory, Joshua prayed to the Lord to stop the sun from setting thus making the day twice as long. My question; I don't doubt that this happened but why was it necessary since the battle was already won? In the end, Joshua had the five kings who had opposed him in the battle killed and he took over all their lands.