Verses 15:13 - 15:16 is a Prophecy to Abram predicting the captivity of the twelve tribes of Jacob (Israel) in Egypt. Since the "iniquity of the Amorites is not yet full" God needs to give the Amorites a fair chance to prove themselves.

When the twelve tribes of Jacob were guided to the land of Cannan after their captivity in Egypt ended, the iniquity of the Amorites must have been filled because most of them were removed from the land that was given to Israel as an inheritance for ever, as long as they walked in the ways of God.

This verse also explains why the tribes of Israel did go into captivity and slavery in Egypt.