purpose is the reason an author writes about a topic.
an author writes to share a main idea is directly related to the author's purpose. In general, authors have three general general purposes of writing: to inform, to entertain, and to persuade.
To inform -to give information about a subject. Authors with this purpose wish to provide facts that will explain or teach something to readers.
Example : pain is a normal part of a physical process that lets us know something is wrong.