Writing does not spontaneously occur. All writing, from a novel to a grocery list, has a specific purpose intended by the writer of the piece. What is purpose?

PURPOSE IS WHY A WRITER WRITES.

Before we can examine purpose, we must examine the intended audience. An audience are the intended readers of a piece. When writers write inside a genre, they are automatically writing to a certain audience (for example, only horror lovers would read a horror novel). Sometimes the publisher or writer determines the audience: for example, an article published in the Journal of American Medicine will be for an audience of doctors, while a book written by John Green is most likely for a teen audience. 

Once you select your audience, you can select what purpose you want to put behind your writing. Here are the different purposes writing can have:

To entertain an audience

A great many texts are made to entertain audiences and sell copies of the text. Most poems, dramas, and narrative genres are made to entertain, such as mysteries, true crime, fantasy, horror, romance, and comedy. Even a text that has another primary purpose has to, on some level, entertain readers enough to keep them captivated throughout the text. 

To inform or teach an audience

Texts with the purpose of informing or teaching are objective or didactic in tone and are based in facts. Though edifying texts like textbooks, essays, and articles come to mind with informative purpose, most mythic texts are also informational, as they explain the origin of something (like the story of Pandora's box) or explain the correct behavior one should have (like the fable of the Ant and the Grasshopper). 

To persuade an audience

Persuasive texts intend to actively convince an audience to believe in the same ideas as the writer. All rhetorical texts and essays are persuasive in nature as they make an argument. Sometimes a narrative or dramatic work can also have a persuasive theme. The key term here is "actively:" any text can theoretically change someone's opinion on a matter, but only texts with an active and obvious intent are persuasive.

To warn an audience

Sometimes persuasion goes beyond trying to change a mind and goes into providing a warning to readers that, should something continue or come to pass, what positive or negative consequences will occur. This is the basis of science fiction writing, documentary film, and political-based narratives and rhetoric. Warning texts are usually about stopping a practice or idea that happening or going to happen.

To call an audience to action

Conversely, call to action texts are usually about starting a practice or idea. A call to action is a persuasive text that goes beyond trying to change an opinion but tries to change how a person acts. Many persuasive texts have a call to action at the end, and all advertisements are a call to action (specifically, "buy X").

To reflect (for self, not audience)

Finally, reflective writing is nonfictional and intended to relive or process a personal experience. This can be written for an audience to read or watch, but is not tailored for that audience at all. Most poems and memoirs fall into reflective writing, as well as live Q and A and interviews. Since reflective writing has no audience in mind, it is usually seen as the most "honest" reflection of a writer from a biographical critical perspective.