Philosophical Literature
This is kind of a no-brainer, since the Republic is nothing if not... you know, philosophical literature. A super important quality of Plato's take on philosophical literature is the fact that it's formatted as a philosophical dialogue, which is really a fancy way of saying "conversation."
Instead of just laying out his philosophical principals in a nice, clear, organized textbook kind of way, he chooses to tell stories about people's conversations, so that sometimes, the philosophical principles can be difficult to find and remember. It also means that we always have to stay on our toes as readers, because there's no overarching narrator telling us "this is true" or "this guy is right." We have to make those calls on our own.
Socrates often seems to contradict himself, and he often makes weird jokes, which suggests that there's more at stake for Plato in the concept of "philosophy" than simply a bunch of doctrines. The dialogue form shows us not just what philosophy is all about; it also shows us how to go about doing it. In that sense, the most important thing in Plato's dialogues isn't necessarily the philosophical conclusions that are reached; it's the process of getting there that's most important.
That fits with Socrates's main concern: getting people to question themselves and their reality. Because searching for philosophical truth is a lifetime endeavor, it makes sense that Plato places a lot of focus on the process itself of questioning and searching.
Philosophy might be more about engaging in thoughtful conversation with your friends than about everyone agreeing in the best or right way to do something. A lot of people have agreed with Plato about this: the dialogue became a very popular philosophical format, especially during the Middle Ages and the Renaissance (hey there, Utopia).