Here's why:
* Purpose/focus refers to the writer's reason for writing. It's the driving force behind the piece, determining what the writer wants to achieve through their writing.
* Different purposes lead to different types of writing. For example, a persuasive essay has the purpose of convincing the reader, while a narrative has the purpose of telling a story.
Other writing traits, such as ideas (the content of the writing), organization (the structure of the writing), voice (the writer's individual style), and conventions (grammar, mechanics, etc.) are important, but they don't directly address the *why* behind the writing.