When I write non-fiction, outlining, for me, is indispensible. It keeps me on track, reminds me of points to highlight, and helps chart my progress. I can use the outline for my Table of Contents, as well as for my proposal.
So, is an outline that important when writing fiction? For me, the answer is "yes." But, the outline I employ for fiction is quite different.
When I start work on a novel, I have the characters and basic idea. I may even have thoughts on a key scene, or several scenes, which is how I begin the outline. I sketch out, with a line or two, each scene in my head. Once I write this down, it is still fluid, not etched in stone.
Usually, having those key scenes make more ideas flow easily. Some I use, some I keep for possible use, some just won't do. I can move scenes, add characters as needed, and start writing. But I never throw any idea away.
And, I don't start on page one. I write one key scene first; once I have that, I can work backward to set it up and forward to its conclusion. That key scene may end up being the opening scene, but I don't think about that when I write it.
So, that's why I start with an outline. If the key scene would fit better at the end of the book, I have an idea of what leads up to it. If it is a scene that should come at the beginning, I have a base upon which to build. My characters will tell me, as I continue to record their story.