The summary of Israel’s history (Joshua 24:1–13) is given to emphasize that God is powerful.
He sent the plagues on Pharaoh (Joshua 24:5). He caused the sea to close in on chariots and horsemen (Joshua 24:7). Og of Bashan and Sihon were given into Israel's hands (Joshua 24:8). Jericho was destroyed, kings of the north and south defeated in battle. The Lord was fighting for Israel and by his power he defeated their enemies and brought them into the land. It was not Israel’s sword or bow which achieved the victory but God’s power. No one can stand against him.
As Joshua wants to encourage Israel to serve the Lord he begins by reminding them that the Lord is powerful and gracious. There is no one like him and there is no one who can stand against him.
1 Joshua gathered all the tribes of Israel to Shechem and summoned the elders, the heads, the judges, and the officers of Israel. And they presented themselves before God.