These tribes are mentioned in order to show that they were busy keeping the commands and instructions of Moses (Numbers 32:20–22; Deuteronomy 3:18–20). These tribes wanted to live in the land on the eastern side of the Jordan River (Numbers 32:1–42) and they were allowed to do so by Moses (Deuteronomy 2:26 – 3:17) on condition that they also fight with the rest of Israel to capture the land to the west of the Jordan River.
12 The sons of Reuben and the sons of Gad and the half-tribe of Manasseh passed over armed before the people of Israel, as Moses had told them.