I have taught Advanced Placement writing for over 13 years and have been a writer for over 25 years. I can tell you that one of the most important skills a writer can develop is an ability to outline. First of all, I want to make sure that the reader understands the difference between outlining and brainstorming. Brainstorming is where we create a myriad of ideas based on a topic in order to formulate other ideas and hopefully some original ideas. Outlining is the activity by which a writer takes those ideas that they developed in brainstorming sessions and organizes them into a coherent structure.
I usually outline the plot of my novel first, with each roman numeral representing an individual chapter. I insert any ideas I have as I go, but this helps me to structure the novel so that all plots and sub-plots are covered and followed to their logical conclusions. I can say from experience that even if you are a writer who places your main character into some type of conflict (whether internal or external) and then allows your imagination to get them out of it without a formal outline, I would argue that you still have some type of rudimentary plan in the back of your mind as to where you want that character to end up. Why not go the extra mile and plan that out on paper so that you can go back and edit it, rework it, before writing fifteen chapters that you have to scrap because you don't like them or they don't end up "working".
The mind is an incredible thing, but if you are like me, remembering all the major plot points and symbolism and other devices can get overwhelming. If you don't outline, you may end up with a more shallow rendering of your vision. Why not record the plot and sub-plots so that you can see the entire text at once? This will allow you to go through it numerous times and tweak it until it is put together more like a fine tapestry rather than a cardboard box.
I have outlined my current novel project. I have a general plot line, but having it written out allows me to see the bigger picture. It has helped me to insert more detail, more allegory, more symbolism and more of the stuff that makes a novel worth reading and worth thinking about.
If you are someone who has fought writing outlines, then give it a try just once. I promise it will be worth the effort. For my AP students, it is often the difference between scoring a 2 (capable of college work) and a 5 (eligible for college credit).