>>24537949 (OP)If we're talking about Plato in particular, we can guess or speculate n a number of reasons. Though, like
>>24537958, I disagree that the dialogues are simply interlocutors saying, "yes, of course, Socrates." Think of the Euthyphro, where Euthyphro himself both has trouble following some of Socrates' arguments, and who, towards the end, even expresses some disdain towards the way Socrates articulates a definition of piety. Same goes for the Meno, where Meno both has trouble following Socrates, and at the same time is used to memorizing what someone else (Gorgias, in his case) explicitly teaches, and so is resistant to inquiring the way Socrates does. And there's all sorts of other moments or dialogues, like the combative Euthydemus or the Gorgias, or things like the complaints and objections of Glaucon and Adeimantus in the Republic.
But as for reasons why the dialogue format:
1) Plato is paying tribute to the man who more than any other turned him to philosophy, and that tribute depicts a man who didn't necessarily always straightforwardly teach something, but who inquired together with others through conversation.
2) Dialogues give Plato a kind of plausible deniability in taking responsibility for certain things said in the dialogues. His friend Socrates was after all put to death, and if an Athenian hostile to the memory of Socrates thought to charge Plato too, he has the excuse, "but where do *I* say these things? It's only Socrates, or Timaeus, or whoever who said them." Observe how dialogues like Phaedo (in which Plato brings himself up to say he was absent), Theaetetus, Symposium, and Parmenides are attributed to the memories or authorship of others, or how Republic, Charmides, Lysis, and Rival Lovers are portrayed as reminiscings of Socrates.
3) It may be that Plato thought that writing treatises was inappropriate to pedagogy towards the activity or way of life that he took philosophy to be, and so he may have considered it important for potential students of the ages to see examples from different viewpoints and circumstances, and with different kinds of people as interlocutors, of how to approach thinking about these topics.