One purpose could be satirical, to show by exaggeration how trivial an accomplishment these rhetorical tours deforce were; better, since Socrates' speech is in fact a highly skilled oration of the genre intended (with all the overblown praise of Athens and the selective attention to history that that entails), is to think it may show (as indeed the Phaedrus claims) how very much better a skilled philosopher is at the composition of speeches than the usual rhetorical 'expert'.