What a platonic dialogue is
At the time Plato sets out to write his dialogues, no cultural tradition in philosophical authorship exists. Previous philosophers had made use of every kind of speech in their attempt to convey their philosophical ideas. It is not certain that Plato was the first to set down philosophical dialogues. We know that Xenophon had scribed Socratic dialogues; we also have extant dialogical excerpts from Antisthenes, Aeschines, Phaedo, Euclid; and also relevant testimonies about Aristippus. Therefore, it is plausible that the genre of philosophical dialogue was formed as an attempt to honor the memory of Socrates - an assumption that derives support from Plato's choice to place the dramatic content of his dialogues in the period when Socrates lived and taught.
The platonic dialogues resemble, first and foremost, theatrical pieces: the dramatis personae are usually historical figures of the 5th c. B.C.; emphasis is put on the scenery; the dramatic time is coherent, thus yielding the initial impression of realism; and the characters use the everyday language of Plato's time. Furthermore, each of Plato's works is autotelic, it doesn't’ hinge on any of his other works, and it doesn't refer -at least directly- to any of them. Therefore they can be read as autonomous works. Distinctly, however, platonic dialogues are exclusively composed of arguments. The dialogical form molds the philosophical argument into a sequence of questions and answers. Instead of representing the mundane conversation of the Athenian agora, the dialogues are designed to illustrates the process of teaching, or dialectical learning.
Given the assumption that philosophy is, for Plato, mainly a "lesson", i.e., an active exchange on vital matters, then only the writing genre of dialogue can bear a similarity to the vivid oral exchange - and, thus, it seems to eliminate, as much as possible, the shortcomings of the written word.