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. Formal outlines are most helpful once your research is complete and you are approaching the rough draft stage. Depending on what type of paper you are writing, you might want to use one of several types of formal outlines. Topic outlines are used for all types and lengths of papers. They list topics and details as single words or phrases and contain subdivisions which follow a parallel structure.