The point of a design-doc (or at least, design notes) for a single developer is simple: it helps you realize your objective with the game and discover any design flaws before you muck up your code with what could be a waste of time. I don't know about you, but I can write up a couple paragraphs far faster than I can whip up a prototype of even a simple idea.
Hmmm - perhaps there's a confusion between design notes and design documents here. I have no objection to design notes, but a design doc (which to me is comprehensive and much, much more than a couple of paragraphs) just doesn't work for me. I'm finding that ideas can be great in theory (on paper if you like) - but just don't play well in practice. I'd rather find that out sooner rather than later. But each to his own.