Legend is told from the first-person perspective of two main characters—Day and June—and the chapters switch back and forth between them. We only get their point of view from their voice in each chapter; so even if we know something from June's chapter, Day's not going to know it in the next chapter. This happens when they're staying together and he has no idea what her name is or where she comes from:
She won't tell me her name.
I can understand that well enough. Lots of kids on the streets of Lake try to keep their identities a secret, especially after participating in something illegal like a Skiz fight. (1.13.1-2)
The first person narration makes it so that we get emotionally invested in the characters; we know what drives them, what makes them suffer, and why they make the mistakes they do. But because we have no more insight than they do, we can't guess what's going to happen in the future. It's all a wild ride for the reader and for our narrators.
The split narration also allows us to see the same event from two different perspectives. For example, when Day's mom is killed, we get his perspective—which is pure shock and rage. But from June's perspective, we get guilt and horror, and because of this we understand how the same event is seen differently from behind Day's and June's eyes.