Genesis 25:6 - Matthew Poole's English Annotations on the Holy Bible

The concubines, Hagar and Keturah. Concubines are sometimes called wives, as Genesis 16:3 Judges 19:1-3,29, but their children had no right to the inheritance. For though the children of Jacob's concubines did equally partake of the inheritance with the other children, that was done by Divine appointment, and Jacob's voluntary act, and upon special reason, because of the vast inheritance promised, and afterwards given to them, which made it no loss, but a great convenience and advantage to the children of the chief wives to have their half-brethren, the sons of the concubines, seated so near to them. Sent them away from Isaac; partly, that the entire possession of that land might be reserved to the children of Isaac; and partly, lest nearness of relation joined with cohabitation or neighbourhood should beget a great familiarity between them, whereby Isaac's seed were likely to be infected by their brethren, whose degeneration and apostacy Abraham might easily foresee from the evil inclinations of their own hearts, and God's exclusion of them from that covenant of grace and life, which was the only effectual remedy against that powerful and universal corruption. Eastward, unto the east country; into Arabia, and other parts of Asia the Greater, which were situate eastward from the southern part of Canaan where Abraham now was, whence these people are oft called the children of the east, as Judges 6:3, Judges 7:12 Job 1:3.