I keep design documents for all projects. They're more like living notes of ideas than concrete goals, and the final product is always entirely different from the vision in the first document. It's immensely useful though since it helps me to remember everything, put it into perspective, remove bad ideas, develop better ideas, and minimize scope creep and complexity.
Having ideas bouncing around in your head really hinders productivity. Once they're written down though you can clear your mind of them and focus entirely on creating instead of just thinking about it.
When parts of the document become uninteresting or hindering, cut them out and figure out something better. Always keep it fresh and exciting. Everything in it should be pushing you forward to your full potential since nobody is gonna be giving any pats on the back for following a plan. They just want something that works and works well, and that's how your document should be for you.