Here are some general, common purposes for writing, with which you are probably familiar:
Inform, Explain, Entertain, Advise, Describe, Argue, Persuade, Review.
This following reponse to a preview of a talent contest from a TV listing magazine demonstrates this:
The wirter's purpose is mainly to pursaude the viewers to watch the programme. The facts about the appearance of the, 'long-awaited return to our screens of…