Purpose dictates the style in which you write, as different styles of writing can convey different meanings to the reader.
For example:
If your purpose is persuasion, you would write in a formal manner, involving rebuttals and counterarguments and whatnot.
If your purpose is entertainment, you would incorporate humor and anecdotes.
If your purpose is to present information, you could use a Context/Quote/Analysis-type format and utilize sources/citations.
