Fair question. Every writer is different. Every writer has varying methodologies to get the story from A to Z, right?
Consider that when building a house, and before you purchase the concrete, wood, nails, pipes, wires, faucets, sinks, countertops, etc – you have a printed floor plan. In black and white it specifies whether it is it a one story or two-story house. Will the kitchen be near the bathroom? Will the doors open in or out? Where are windows? If there are stairs, where they are positioned. You get the drift.
An outline is much the same. It’s the structure around and for your story. The framework, so to speak. That doesn’t mean that you must absolutely follow what you indicated in your outline. The outline should help clarify if you’ve included pertinent aspects of your story.
Imagine a cozy mystery, where an old man is murdered during a town celebration. He’s curmudgeonly, and most of the townsfolk have reason to dislike or hate him. In the days prior to his murder, several individuals had some known argument or public disagreement with him. The heroine is the town busybody who owns the corner market and a volunteer detective. Of course, she suspects at least three individuals of murdering him.
There are layers, then, to this story. Before the old man was curmudgeonly, he might have been a nice guy, maybe married happily, until some event turned him sour. How does our town busybody factor into the story? And, we need to know a little bit about all the players suspected of his murder. You, as the writer, need to string us along, so that we aren’t quite sure who murdered the old man, until the heroine pieces it all together in the last chapter or two. If the reader figures out in Chapter 2 or 3 who murdered him, then they aren’t likely to continue reading the book.
Without an outline, it is entirely possible that the writer will have continuity issues within their story. Without a framework, the story may meander, go on for a few chapters on some tangent, while meanwhile losing the reader. And as a writer, having to cut unnecessary chapters after having written the whole story, is a bit sad.
Every story has some sort of intentional structure. An outline can keep the story moving – in the right direction.