Why layers instead of a paragraph
An image model does not read your prompt the way you wrote it. It resolves broad structure first and fine detail last, which is why an instruction buried mid-sentence often gets overruled by something earlier.
Ordering a prompt the same way makes it legible. You stop asking why is this wrong and start asking which layer is wrong, and those are very different questions in terms of how quickly you can answer them.
Nothing in your prompt is rewritten in the process. Your wording, mood, and the relationships between subjects survive intact — only the order changes.