Genesis 25:3

And Jokshan begat Sheba, and Dedan. And the sons of Dedan were Asshurim, and Letushim, and Leummim.

And Jokshan begat Sheba and Dedan, and the sons of Dedan were Asshurim and Letushim and Leummim.
They were children of Abraham in his later life through Keturah, Abraham's last wife, and they became peoples of note who grew into large related family circles, clans, sub-tribes and tribes.
The verse falls within the account of God's providential dealings with Abraham's family, the continuing Genesis record of the chosen family line through which the Seed would come.
Within that family the promises extend from Abraham to Isaac and from Isaac to Jacob, and this Keturah line is distinguished from the special, chosen line to which the promise applied, with Isaac alone identified as the promised seed through whom the promise would continue.

The continuing story shows the gradual development of faith in Abraham together with Isaac and his two sons in their defects, leading to Jacob's victory which made him Israel, prevailer with God.
Though these Keturah descendants are fleshly descendants of Abraham just as truly as Isaac and Jacob, they are not reckoned among Israel as entitled to the promises.
Descent from Abraham according to the flesh does not by itself make anyone an Israelite heir, and some other principle besides lineage decides who are the true Israel.

In patriarchal life outside walled cities the family is the real unit of protection, trade and safety, with nearer and more distant relatives uniting against a common foe while still quarreling among themselves.
Having many sons gave strength and protection and avenging of blood, which explains why the patriarchs from Abraham on desired many sons.
The listing of children by different wives shows Oriental polygamy in practice and the different esteem in which different wives were held.