It was to reassure those who would become joint heirs with Christ, making up the secondary part of the promised “seed, ” and to strengthen their faith.
وَلِمَاذَا حَلَفَ ٱللهُ لِإِبْرَاهِيمَ أَنَّ وُعُودَهُ سَتَتَحَقَّقُ ؟‏