Simply put, professional writing must make sense. Writing an outline ensures a well-written piece of work, no matter the genre or purpose, flows well, makes a statement, and most of all, is clear.
One of the greatest difficulties of writing is the concise and relevant presentation of one's ideas. Clear writing, however, represents good preparation; taking the time to construct an outline is almost always reflected in a polished, accessible writing project.
Attempting to write without an outline is like painting a house without first removing the old paint: it will look all right for a little while, but fairly soon the chips and cracks will show. Before setting pen to paper for the opening sentence, it is imperative to ask: What is my main idea? How can I support my argument? What evidence do I have? How will I conclude?
A good writer always knows how the piece might end; a mediocre writer views the writing process as an exploratory journey with a surprise at the end of it.
A strong outline guides the writer along a clearly demarcated path, allowing little room for unnecessary excursions. Often, a writer becomes side-tracked by a peripheral or tangential point (which can often be interesting, but unnecessary); an outline helps keep the writer on task.
A writer who works without an outline can be tempted to overcompensate for poor planning with elaborate explanations or by using a free hand with the thesaurus.
Remember, professional writing is writing you don't notice. It is simply a conduit for ideas-not a showcase for flowery language.