Whether consciously or not, every writer is in search of a process.
The outline is an often misunderstood element in the creative process. Some writers eschew it completely, considering it to limit their imagination, while others set out to plot their stories so completely there will be no surprises along the way.
As I see it, the purpose of the outline is to help writers see beyond their fixed ideas to a more dynamic version, before losing themselves in the world of the story.
The other reason is that it could be helpful to develop a sense of a beginning, middle, and ending before starting the first draft. This often helps free the writer from unnecessary anxiety.
With a basic confidence in the story, this can paradoxically allow the novelist, memoirist, or screenwriter to pursue a narrative with abandon, allowing the story to have a sense of surprise.