At the moment writing for online or letter (including email) communications is the type of writing at the forefront of my mind simply because it is what I most recently used.
The reality is that there is more than one way to go about writing any piece. Many students write on assignment. Some freelancers also write on assignment but are paid. In such cases the structure is provided by others, and the writer simply strives to make whoever gave the assignment happy with his or her work.
There are many writers who create their own structure but plan it in advance. In other situations, people simply write and see what develops from there. Sometimes they have an idea but are not certain of all the details.
There are writers who require their “Muse” and others who require their “Paycheck” in order to do their writing. Others work best under the deadlines of their teachers or the encouragements or threats of their parents or school teachers.