Most probably, the author of Joshua reminds his readers of the land to the east of the Jordan because he wants to reinforce a theme of national unity.1 Towards the end of Joshua (Joshua 22:21–29), we are told that the eastern tribes (Reuben, Gad, and Manasseh) feared a day when they would no longer be considered as part of Israel.2 By mentioning those areas of land here, the author links the conquests of Moses with the conquest of Joshua, showing that together they form a single action of the Lord.3
1 Now these are the kings of the land whom the people of Israel defeated and took possession of their land beyond the Jordan toward the sunrise, from the Valley of the Arnon to Mount Hermon, with all the Arabah eastward: