There are several reasons. Now, I must admit-and this probably makes me sound crazy (which I am)- I don't really study, let alone review. However, the outline is suppose to help guide you throughout the essay. Say you have a question about an essay, well, pretty much all of your answers should be on that outline. An outline also helps you to build ideas, stick them together, and then add in between. Placing the most important things first, and then the least important things toward the end of the essay. The outline can always help you to do this.
