‘ And when they came to the region about Jordan, that is in the land of Canaan, the children of Reuben, and the children of Gad, and the half tribe of Manasseh built there an altar by Jordan, a great altar to look to.'
This building of a memorial altar, in the land of Canaan west of Jordan, was imitated by Gideon later (Judges 6:24). The intention of it was in order that it might be a reminder that the Transjordan tribes were one with those in the land of Canaan and shared in the tribal covenant. It was a gesture of praise to God and of unity with their brother tribes. In a sense this was their possession in Canaan. As they looked at it across the Jordan it would be a reminder that they were one people in the covenant, sharing God's land.
“The region about Jordan.” Or more literally ‘the circles (geliloth) of Jordan', thus a specifically recognised district, possibly based on the circular twisting of the river like a serpent at this point. Possibly by building the altar in a place where the Jordan wound round it on three sides they saw it as on joint territory. Compare Genesis 13:10 where the southern part of the Jordan Rift valley is called ‘the circuit (kikkar) of Jordan'.
“A great altar to see to.” The altar was large so that it could be seen at some distance, and is purpose was so that it could be looked at from Transjordan. It was built on the pattern of the altar in the Tabernacle (Joshua 22:28).