Why might Plato go to such lengths to show that justice is beneficial in itself? The answer may lie in the historical circumstances of his lifetime. Plato was probably born in 428 B.C. and died in 348 (Grube, viii). During his lifetime, Athens was fighting in the Peloponnesian War against Sparta. In 416, Athens seized the neutral island of Melos, demanding that the Melians pay tribute to Athens. When the Melians refused to submit, Athans ravaged the island. Such brutalities may have motivated Plato to think about ethics.
Indeed, Thucydides portrays an imperialistic and unjust Athens in his History of the Peloponnesian War. The Melian dialogue in the History contrasts the demands of expediency and justice. On the one side, the Athenians assert that the Melians should join them because it will benefit Athens, and Athens is the stronger of the two nations. On the other side, the Melians argue that it is unjust for the stronger to force the weaker to do whatever it desires. The Melians determine that it would be better to die free than to live in submission to Athens, much as Athenians chose when Persia invaded Greece. In the end, Athens destroys Melos.
Consequently, Athens was experiencing real ethical dilemmas and tensions during Plato's lifetime. For someone who thinks that virtue is good of its own accord, incidents such as the destruction of Melos would be deeply problematic. Plato's thesis in the Republic concerns the inherent goodness of doing virtue, and so it seems to be the case that Plato would have found Athen's imperialistic conquests to be unethical. Perhaps it was in an attempt to persuade other Athenians to the true path that Plato wrote the Republic.