My conscious brain is a very visual thing, and it can hang onto a lot at once when I'm seeing a story unfold in my eye. It's like Cinemascope. Except you can't write in Cinemascope. You can write broadly or narrowly, but you only have sentences chained together to form your vision. Each sentence feels like a spotlight, even when you're writing broadly, and you have to point the spotlight correctly or the scene goes astray.
I've got eleven major characters with speaking roles crammed in one room with forty other extras and a band. It's the scene that sets up the entire rest of the story, and everybody has to be kept straight. If I had to think about what I want to accomplish in this scene, try to plan it out, use my conscious visual brain, I'd go nuts. This is where I rely on the subconscious's in-the-can version playing in my head. Which, yes, is different from what my conscious brain sees. My conscious brain sees the sets and the cameramen and actors and lights and marks on the floor. It wants to shout orders and play director. The subconscious shows me the 'finished' product. It already has the camera angles, the focus, and I trust it to be what the story needs.
And, of course, there's always the blessing of re-writes, which are usually not so much re-envisioning the scene, but refining how I describe what the subconscious shows me. It's odd to me sometimes how much the physical act of writing a story has little to do with the words. That probably makes little sense, even to other writers, but it's how I work.