in the book outsiders we just learn about pony's reoccurring nightmare? why does he include this in his story

Often a character will include something that is bothering them or something that is recurring in order for the reader to understand why the character acts the way they do or to get a sense of why they are feeling a certain way. In this case, this may also be a foreshadow into what might occur later in the novel. Either way, elements like these add to the tone (the author's attitude) and the mood (the readers feelings) about the text, and allows the reader to start to feel certain ways towards Pony. It also aids in the characterization if the novel.