The writing purpose of the writer should communicate thoughts and feelings with conviction and enthusiasm and provides the overall design that is to govern what the writer has set out to do.
For example, you want to write about pollution. Why do you want to write about it? Do you want to inform the reader of some important but little known facts concerning pollution? Do you want to describe what the city looks like under the smog? Maybe you want to communicate or share some strong feelings about the dangers posed by excessive pollution. Knowing why you want to write about a subject will help you to focus and will lead you to organize the data to suit that purpose.
When you sit down to write, ask yourself: who am I addressing? Who do I want to reach? It is good to remember that all writing must be directed to a specific audience, for instance, your teacher, a hobby club, a magazine or newspaper editor, or a group of students like yourself Addressing a specific group of readers will give your writing an immediacy that will catch and hold their interest.