This was probably a holdover from Macedonian practice and seems to have been a relatively fluid system. Most kings didn’t designate a primary wife and neither wife nor son’s prominence was certain. A favored wife could find her power given to another. A son who was named heir presumptive could see that position taken from him by his father and bestowed on another. Half-brothers scrambled to prove each other illegitimate. Infighting was semi-constant and power never secure.
Why would a king risk that sort of instability? For several reasons. Having multiple wives allowed for the possibility of multiple marriage alliances with both local families and foreign powers. It also meant that if one wife didn’t produce any male heirs, another might. The potential for infighting was preferable to the lack of an heir.
What about the women themselves? Marriage alliances could benefit a wife almost as much as they did her husband, especially if her family was physically nearby. If she had a son, she could rise to become the king’s most important wife and then maybe even the next king’s mother, both positions of significant power. Even if a wife had no sons, she could ally herself with one who did and obtain power and prestige that way.
Some examples are pretty clear-cut. Phillip II had seven wives, the most famous of whom, Olympias, was the mother of Alexander the Great and a fierce political player herself. Ptolemy VIII married both his sister Cleopatra II and his niece Cleopatra III. The marriages of Antiochos II first to Laodike I, then to Berenike, follow more in terms of serial monogamy, but his return to Laodike might share continuity with his predecessors’ practice.
Pomeroy, Sarah. Goddesses, Whores, Wives, and Slaves: Women in Classical Antiquity. New York: Pantheon Books, 1995.
Shipley, Graham. The Greek World after Alexander 323-30 B.C. New York: Routledge, 2000.
Carney, Elizabeth Donnelly. Women and Monarchy in Macedonia. University of Oklahoma Press, 2000. [Can be found on Google Books here.]
Review of Ogden, Polygamy, Prostitutes, and Death: the Hellenistic Dynasties - Bryn Mawr Classical Review