Purpose
Purpose is the reason for writing: What the writer seeks to accomplish.
Resources:
Writing@CSU
"Writing Guides: Understanding Purpose." <http://writing.colostate.edu/guides/processes/purpose/>.
Links to Definition/Types/Strategies/Audience and relationships to other elements of writing.
Why write?
"Writing with a Sense of Purpose"
http://grammar.ccc.commnet.edu/grammar/composition/purpose.htm
The Writer's Motivation
"Purposes in Writing"
http://web.mst.edu/~gdoty/classes/concepts-practices/purposes.html
Writing to Persuade
An interactive map that can be printed
http://www.readwritethink.org/files/resources/interactives/persuasion_map/