Knowing and being able to identify the purpose of a text or reading passage is essential to the comprehension and the understanding of the entire text. At times, the purpose of a text is referred to as authorial intent, which means the author's intent or intention.
Why does author's purpose matter? It matters because understanding not only the "why" the author wrote the story but the "how" the author wrote the story tunes us in to what we should know by the end of the text. Essentially, understanding the reason behind the writing will help with the understanding of the writing. It gets your mind focused on what you will be reading. For example, if you are thinking you are going to hear an entertaining story and it is informational, you may be distracted using words or disappointed with the content, and this will make you miss the message or story completely.