It is helpful to create an Outline for the following reasons:
1) Organize your thoughts/ideas better
2) Understand information flow - logical ordering of information
3) Shows relationships between ideas
4) Ensures that you include everything - helps to keep track of a large amount of information
5) For creative writing - organize plot threads and keep track of character traits
6) For oral presentations - helps you speak more effectively
OWL at Purdue Online Writing Lab
Purdue Owl - Developing an Outline