“Concubinage was practiced in many ancient cultures, especially in Mesopotamia…where a private citizen might have one or two concubines in addition to his primary wife…A concubine was often a slave or part of the booty of war (Judges 5:30). A man might have a concubine simply as an economical form of marriage, since no dowry or bride-price was required. A concubine could add to a man’s prestige by giving him two wives and thus an increased capacity for children. Such offspring were normally delivered onto the knees of the legal wife, thus establishing their legitimacy as family members. The concubine was also another servant to add to his work force.” Baker Encyclopedia of the Bible
1 Chronicles 3:9 These were all the sons of David, besides the sons of the concubines, and Tamar their sister.