Documentation is there to help you understand your project now. If you’re using documentation only as a historical reference then you’re using it wrong.
When you write your thoughts down, you can then read them back to yourself. Then you can see all the flaws and gaps in your thinking. Then you can edit your document to refine your thinking. And if you work on a team, you can then share this best-representation of your thoughts with your colleagues and get their feedback. If you're not doing this then you're almost certainly not creating optimal designs.
Maybe you think this is all overkill, and maybe it is some of the time, but I can't tell you how much stupid code I've read that was written by smart people who weren't thinking clearly.
Using documentation to clarify concepts as I go is certainly aomething that happens, but it is not the reason i do it.