The myth of Er is essentially the 411 on the afterlife. Just like the myth of metals, it's a "noble lie" meant to convince people who don't, or can't, make their way through the Republic that living a just and thoughtful life is the way to go. It tells the story of a man named Er who comes back from the dead and reports on everything he saw there: "And when [Er] himself came forward, [the judges of the dead] said that he had to become a messenger to human beings of the things there, and they told him to listen and look at everything in the place" (614d).
In the underworld, Er sees how greatly just living is rewarded and how badly unjust living is punished. He also sees how, since there's reincarnation, you can end up with a really bad next life if you don't have the proper judgment to choose a good one. In other words, Plato is concerned that not everyone will buy, or have the intelligence to understand, his fancy-pants philosophical explanations for the benefit of justice. So what does he do? He tells a scary story about the underworld to try the fear-as-motivation route.
We haven't forgotten how down on poetry Socrates constantly is, but seriously, one of the awesome things about the myth of Er is that it's a poetic—we might even say epic—way to end this dialogue. You know what epic heroes always do? Yep, go the underworld. (Check out the Odyssey and the Aeneid if you don't believe us).
We think Plato might be just a little bit jealous of all the attention the epic gets... and maybe, just maybe, he wants to channel some of that flash and fame toward philosophy. See, philosophy can involve cool stories about heroes, too. Right?