The entire story of The Witches comes from our nameless narrator. He speaks in the first person because he is, after all, telling a story that has happened to him. Once in a while, especially toward the beginning, our narrator slips into second person, calling the reader "you" – like when he kindly informs us that, "[f]or all you know, a witch might be living next door to you right now" (1.24). Thanks for the warning, guy we just met! For the most part, though, the narrator just tells his story, without referring to us.
This kind of narration works for a few reasons. First, we get to hear the story from the point of view of a kid. Can you imagine reading this same story from the perspective of an adult? They wouldn't be in danger of witch squelching, for one thing, so it wouldn't be quite as exciting or scary or exhilarating. The first-person narration adds to the childlike wonder of the whole thing. Also, it gets us as close to the story as we can possibly be. The person who lived it is the one telling it. Especially in a story about witches – something we (hopefully) haven't experienced ourselves – it really helps to have someone who has experienced it telling us the story.