Outlining helps you find strong and weak areas in your research. It reveals imbalances and inadequately developed topics. It forces you to remain relatively reasonable and logical within the scope of your essay. Outlining will reveal which of your points is insupportable by evidence that is understandable to your reader. It reveals which topics and details are unnecessary within your paper.
Outlines separate the junk from the good stuff.
Outlining stimulates more thought about your thesis. By revealing strong and weak points, outlining may generate new ideas about your thesis. Outlining forces you to rethink every topic and detail in your paper. Informal outlines are most helpful for preliminary planning. They create order out of the chaos within your brain without getting too specific or restrictive.