"(The Lord had done the same for the descendants of Esau, who lived in Seir, when he destroyed the Horites from before them. They drove them out and have lived in their place to this day. And as for the Avvites who lived in villages as far as Gaza, the Caphtorites coming out from Caphtor destroyed them and settled in their place.)" NIV translation
Why are the details like this included in Moses' parting words to Israel? We, like the Israelites, are strengthened in our faith as we see God do things in our lives. But, what do you do when you do not have those past experiences? You hear the testimony of others and what God has done in their lives. This, too, can strengthen you and help you stand strong in facing your giants.
The Israelites were on their way to face their first battle. As they passed through the lands of their relatives and saw what God had done for them, They would have had a stronger faith and hope that He would be with them. Moses reminded the people that it was God that had done the work and secured the victories.
"'Set out now and cross the Arnon Gorge. See, I have given into your hand Sihon the Amorite, king of Heshbon, and his country. Begin to take possession of it and engage him in battle. This very day I will begin to put the terror and fear of you on all the nations under heaven. They will hear reports of you and will tremble and be in anguish because of you.'" NIV translation
The Arnon Gorge is in the modern country of Jordan where the locals call it the "Grand Canyon of Jordan". At this time, it was the boundary between the land of the Ammonites and the Amorites.
We see that, before they crossed this massive gorge, God assured them that He had already gave them the land on the other side. They had already passed through lands that God had taken from others and given to their relatives. Because of that fact, they would have had a stronger faith and trust in the Lord.
We also see that their victory was going to be a testimony of God's power to the nations of the world and a reminder that, as the Creator of all things, He has the right to set the boundaries of the nations.