An outline is a formal system used to develop a framework for thinking about what should be the organization and eventual contents of your paper.
An outline helps you predict the overall structure and flow of a paper.
Writing papers in college requires you to come up with sophisticated, complex, and sometimes very creative ways of structuring your ideas.
Taking the time to draft an outline can help you determine if your ideas connect to each other, what order of ideas works best, where gaps in your thinking may exist, or whether you have sufficient evidence to support each of your points.
University of North Carolina; Reverse Outlines: A Writer's Technique for Examining Organization. A good way to check your progress is to use what you have written to recreate the outline.
Indiana University; Writing: Considering Structure and Organization. How do you know whether to change the paper to fit the outline, or, that you need to reconsider the outline so that it fits the paper?
Writing an outline helps you ensure that your writing stays focused, and it can help you to organize your ideas so that they make the most impact.
For example, it may be necessary to include a strong supporting detail right at the start to grab readers' attention, or you may find that a strong detail is best left to the end so the essay can go out on a bang.
The sentence outline is also useful because sentences themselves have many of the details in them needed to build a paper and it allows you to include those details in the sentences instead of having to create an outline of short phrases that goes on page after page.
A strong outline details each topic and subtopic in your paper, organizing these points so that they build your argument toward an evidence-based conclusion.