I am embarrassed to say I learned this writing principle the hard way. It was my first semester of my freshman year, and my English professor assigned a paper on a poem that I didn't like. As the due date approached, I sat down and composed three pages of text--even though I had nothing to say. I marveled at my abilities.
The awakening came when the paper was returned with a low grade and the comment, "you've written three pages and said nothing." Of course, I had, but so what. My sentences were well-constructed. My paragraphs had unity and cohesion. I had no typos or errors in mechanics. Such a big deal made for missing one ingredient: a purpose.
I made an office visit to Professor Jeanne Braham who then instructed me to begin with the end in mind. What did I want my reader to think, feel, understand, or do after he/she read my piece? Professor Braham taught me unless you can state your purpose, or controlling point, in one (or maybe two) concise sentences, you aren't ready to write. Sometimes this means you have to go back and research or learn more about your topic (what I should have done), or sometimes you have too many unrelated points and you have to identify your primary purpose, and then limit your scope. But before you can begin, you have to clearly identify your purpose.
Two of the most important writing principles are audience and purpose. Lose touch with either and the force will not be with you!