"One theory provided by Herodotus is that Leonidas sent away the remainder of his men because he cared about their safety. The King would have thought it wise to preserve those Greek troops for future battles against the Persians, but he knew well that the Spartans could never abandon their post on the battlefield. The soldiers who stayed behind were to protect their escape against the Persian cavalry. Herodotus himself believes that Leonidas gave the order because he perceived the allies to be out of heart and unwilling to encounter the danger to which his own mind was made up. He therefore chose to dismiss all troops and save the "glory" for the Spartans."
Note that "glory" is in scare quotes. Oooh, how wise and post-modern.