Theophilus gives two reasons: Theological, to prove that Adam and Eve were not created by different gods. Anthropological: increase their affection:

And Adam having been cast out of Paradise, in this condition knew Eve his wife, whom God had formed into a wife for him out of his rib. And this He did, not as if He were unable to make his wife separately, but God foreknew that man would call upon a number of gods. And having this prescience, and knowing that through the serpent error would introduce a number of gods which had no existence,—for there being but one God, even then error was striving to disseminate a multitude of gods, saying, “Ye shall be as gods; ”—lest, then, it should be supposed that one God made the man and another the woman, therefore He made them both; and God made the woman together with the man, not only that thus the mystery of God’s sole government might be exhibited, but also that their mutual affection might be greater.

Theophilus of Antioch, “Theophilus to Autolycus,” in Fathers of the Second Century: Hermas, Tatian, Athenagoras, Theophilus, and Clement of Alexandria (Entire), ed. Alexander Roberts, James Donaldson, and A. Cleveland Coxe, trans. Marcus Dods, vol. 2, The Ante-Nicene Fathers (Buffalo, NY: Christian Literature Company, 1885), 105.