An extract from Kathryn A. Morgan may help elucidate the rationale of the Atlantis myth :
The myth of Athens and Atlantis in Plato's Timaeus and Critias can be, and has been,
interpreted on a number of different levels. On the most fundamental, philosophical level the
myth sets into narrative motion the paradigm of the ideal state elaborated in the Republic. Gill,
in a series of publications, has done much to throw light on the nature of this invention: its
relationship with modern categories of fiction and with antecedent historiography. Yet the extent
to which the myth of Atlantis is embedded in larger fourth-century political and historiographical
concerns has been insufficiently appreciated. In what follows, I shall attempt to reconstruct
some of these concerns. I shall argue, first, that the narrative set-up of the Atlantis myth
corresponds to the conditions specified in the Republic for the successful creation of a charter
myth (the 'Noble Lie') for the ideal city, and that this is a valuable indication of the truth status
of the myth and of the function it is expected to perform. This function is not merely a matter
of abstract philosophical interest, since there are close parallels between the Atlantis myth and
contemporary panegyric versions of Athenian history. ...
The contents of the myth are well known. At the beginning of the Timaeus, Socrates declares
his dissatisfaction with a Republic-like discussion held on the previous day (19b-c). He wants
an account of the just city in action, rather than a bare description. The other interlocutors in
the dialogue are the astronomer Timaeus (probably a fictional character), and two historical
politicians, Critias of Athens and Hermocrates of Syracuse. The philosophical entertainment envisaged by the interlocutors is that Timaeus will give an account of the creation of the
universe, followed by Critias, who will tell the story of ancient Athens and Atlantis. By a happy
coincidence, the excellence of ancient Athens is well-suited to set the picture of the ideal state
into narrative motion. The narrative of the Timaeus tells how
the city that is now Athens was the best in war and had in all respects the best laws by far. It is said to
have performed the finest deeds and to have had the finest constitution of all of those we have heard report
of under the face of heaven. (23c4-d)
Among these deeds, the greatest was the defence of Europe and Asia against the hybristic island
empire of Atlantis. When Atlantis attacked Athens showed its excellence. She was the leader
of the Greeks, but when they all deserted her she stood alone, defeated the enemy, prevented
the free from being enslaved, and freed those who had been. This victory was, however,
followed by earthquakes in which Atlantis sank into the sea and the Athenian army was
swallowed up by the earth (24d-25d). The narrative of the Critias recounts in greater detail the
disposition of ancient Athens and Atlantis, and a a begins to tell how Atlantis declined from its
ancient virtue before the dialogue breaks off. Here then indeed is a charter myth for Plato's
Athens. The Egyptian priests who are the ultimate source for the tale narrate that the laws of
ancient Athens enjoined a strict hierarchical system, with priests, warriors, artisans and peasants
(Tim. 24a-b). Critias explicitly identifies this system with that of the ideal state in the Republic. The citizens whom Socrates spoke of on the previous day are in fact Athenian ancestors (Tim.
25e-26d). (Kathryn A. Morgan, 'Designer History: Plato's Atlantis Story and Fourth-Century Ideology', The Journal of Hellenic Studies, Vol. 118 (1998), pp. 101-118 : 101-2.)
In other words, the ideal city (kallipolis) sketched in the Republic had a charter myth, the Myth of the Metals, a gennaion pseudos (Rep. III.414b) which is not best rendered as 'Noble Lie' but as the metaphorical or mythical statement of a truth which the general populace can understand and be persuaded to accept only in some such form. Plato is suggesting that in real history Solon's ancient Athens had qualities similar to those of the kallipolis. To match the Myth of the Metals in Plato's speculative sketch of the kallipolis in the Republic, the Athens of Plato's day is given a charter myth in the Atlantis story; from the destruction alike of Atlantis and ancient Athens, the Athens of Plato's own time emerged.
The assurance of both Citias and Socrates that the Atlantis story is true (Tim. 20d7-e1) is a puzzle. Christopher Rowe regards this as an example of Platonic irony (5 C.J. Rowe, 'Platonic irony', Nova Tellus 5 (1987). This is quite as good an explanation as any.