Here's why:
* Context: Understanding the historical, social, cultural, and political environment in which a document was created helps you interpret its meaning and significance.
* Author's Purpose: Knowing why the author created the document—to inform, persuade, entertain, express personal feelings, etc.—gives you crucial insights into the document's intended message and potential biases.
In short: To fully understand a primary source, you need to consider both the context in which it was created and the author's purpose in creating it.