I confess. I'm a plotter.
Part of it's out of preference, part out of necessity. Anymore, when I turn in a manuscript, editors want an outline. Or they want an outline as a part of my contractual obligations, even before I set a word to paper. They want to know the hell I'm cooking up, so that there will be no surprises. That's the necessity.
As far as preference goes...I dislike staring at a blank page. It's intimidating. I want to have some idea of where I'm going and how I'm gonna get there.
I begin with a high-level outline. A skeleton or scaffolding. As I work through the manuscript, it becomes more detailed. Flesh gets added to the bones. There are ideas that need to be reiterated, loops that need to be closed, threads to tie up. It eventually breaks into a scene-by-scene outline.
The scene-by-scene outline allows me to easily create a timeline (another frequent editorial request). I find that I'm less tempted to try to pack a superhuman number of events into my heroine's day if I have a visual representation of how much stuff I'm trying to cram between sunrise and sunset.
Breaking a story into scenes helps me to control chapter lengths. If I scribble down the gist of one scene POV, and the number of pages on a notecard. I can mix 'em up and put them together in many configurations. Keeps me from getting too wedded to a certain order and keeps me questioning breaks and POV shifts. I also try to write down on the notecard the purpose of the scene. If I can't come up with at least three, it goes into the trash bin.
As you may have guessed, my outline starts out small. At the outset of a project, it may be only three or four pages. But, as the project grows, I faithfully record what I'm doing on paper and cards. When I'm done, I have a detailed outline that I can analyze for pacing issues, logic gaps, and general WTFckery.
That's not to say that I have no serendipities, no flow. I do chase ideas down rabbit holes and find my own little synchronicities. The outline is not sacred - it's meant to be torn apart and reconstructed.
But I like having a light in the darkness to show me where I'm going.