For an excellent article on why outlining is important, read the entire entry through this link.The first paragraph:
Generating an outline before working is not just a good idea, it is essential. Most of the thinking that goes into an essay should be done by the time the outline is written, so that when you actually sit down to write your essay from the outline, your writing is not a matter of formulating ideas in your mind but merely a matter of presenting those ideas in a coherent and persuasive manner.
