How Audience and Focus Affect Purpose
All readers have expectations. They assume what they read will meet their expectations. As a writer, your job is to make sure those expectations are met, while at the same time, fulfilling the purpose of your writing.
Once you have determined what type of purpose best conveys your motivations, you will then need to examine how this will affect your readers. Perhaps you are explaining your topic when you really should be convincing readers to see your point. Writers and readers may approach a topic with conflicting purposes. Your job, as a writer, is to make sure both are being met.