Matthew Poole and continuators on Genesis 25:6

6 But unto the sons of the concubines which Abraham had, Abraham gave gifts, and sent them away from Isaac his son, (while he yet lived) east-ward unto the east-countrey.

Hagar and Keturah. Concubines are sometimes called Wives, as Gen. 16. 3. Iudg. 19. 1, 2, 3. 29. but their Children had no right to the inheritance. For though the Children of Iacobs Concubines did equally partake of the inheritance with the other Children, that was done by divine appointment and Iacobs 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.

Partly that the intire possession of that Land might be reserved to the Children of Isaac; and partly lest nearness of relation joyned with cohabitation or neighbourhood should beget a great familiarity between them, whereby Isaacs 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 Gods exclusion of them from that Covenant of grace and life, which was the onely effectual remedy against that powerful and universal corruption

Into Arabia and other parts of Asia the greater, which were scituate east-ward from the Southern part of Canaan where Abraham now was, whence these people are oft called the Children of the East, as Iudg. 6. 3. and 7. 12. Iob 1. 3.