Method - Purpose
Purpose - This is WHY we write.
Authors have a reason to write. This is the message, as a Main Idea within a paragraph, or as a  
Thesis, the controlling ideas in an essay.

Method - This is HOW we write.
This how authors chooses to develop and shape ideas in individual paragraphs or entire essays.
 
The following list connects various purposes you may have for writing and the methods for achieving those purposes. Examples of a purpose are provided to give an idea of how these two concepts differ.
 
Narration
method
To tell a story about your subject, to enlighten the reader or to explain something to them
example of a purpose
(to show a concert can be good for a town)
The concert at the stadium last night was awesome, and really brought the people of town together for a positive event for a change.
Description
method
To help readers understand your subject through the evidence of their senses: sight, hearing, touch,smell, taste
example of a purpose
(to emphasize the hazards of drunk driving)
That accident was horrible; it resembled a battlefield, and all because of a careless drunk driver.
Example
method
To explain your subject with instances, to show readers its nature or character
example of a purpose
(to present an example of what young teens should not look up to)
Madonna is a very poor example of a role model for teens; her dress, smutty vocabulary, and personality are not to be envied.
Comparison\Contrast
method
To show similarities and differences
example of a 2 purpose
(to make a choice & show cooperation)
To arrive at a preference, the author's choice
After careful consideration, it appears that a Toyota Camry is a much better car for the money than a Honda Civic.to To show how 2 subjects work better together.
Frank is a great hoop shooter, and Mike is the best rebounder  on the league; together they brought their team to a victory.
Process Analysis
method
To inform readers how to do something or how something happened-to let readers see how something works, how a sequence of actions leads to conclusion
example of a purpose
(to say that rock stars really do not need musical ability, only showmanship)
To become a rock star, al that is necessary is that a performer dress really flashy, dance like a wiggling snake, and know about three chords on a guitar; musical talent is optional.
Division\Analysis
method
To explain a conclusion about your subject by showing the readers parts and elements
example of a purpose
(To show that a car is really an extension of a man's masculinity)
A Corvette is designed to induce older more established riders to purchase it because of it many options that make a driver feel cool and young.
Cause\Effect
method
To tell readers the reason for consequences of your subject, explain what made it happen or what was the result
example of a purpose
(to illustrate that just how bad smoking is for a teen)
Smoking is not only a filthy habit, but can create unnecessary aging, and social problems for teens.
Definition
method
To show readers the meaning of your subject-its boundaries, its distinctions from other subjects
example of a purpose
(to shock a reader into viewing smoking as an addiction, not merely a habit)
Smoking is a drug addiction, and those who engage regularly in this activity are drug addicts.
Argument\Persuasion
method
To have readers consider your opinion about your subject or your proposal for it, to advance your position
example of a purpose
(to let a reader see just how annoying a disruptive children can be)
Children under the age of 12 should not be allowed in restaurants after 9 o'clock at night because they often create an environment of tension and aggravations.