Why create an outline? There are many reasons; but in general, it may be helpful to create an outline when you want to show the hierarchical relationship or logical ordering of information. For research papers, an outline may help you keep track of large amounts of information. For creative writing, an outline may help organize the various plot threads and help keep track of character traits. Many people find that organizing an oral report or presentation in outline form helps them speak more effectively in front of a crowd.
(from Purdue OWL, How to Outline)