Beyond making the suits happy, good documentation really can help make your game better, regardless of whether you are developing it alone in your basement or with a team of thirty other developers. I have listed a number of separate documents above, and over the course of the project it may become difficult to keep them all straight, organized, and accessible to the team members who need them at any given time. To assist with this, some developers have started using Wiki, a web-based system for organizing documents and allowing members of the team to easily modify or update any document, all while maintaining version control and history for each file. Though not without its drawbacks, aWiki system will allow you to keep your documents organized while also allowing them to be kept up to date as the project changes and evolves.
As a game designer, you should be involved and interested in the creation of all of the documentation described above. As a lead or senior designer, the creation and maintenance of the design document, story bible, and script are all your responsibility. Each document may be written by an individual or worked on collectively by a number of people. For example, you may not actually write the script yourself if there is a writer available more qualified to compose compelling dialog. Yet as the lead designer, you must still be concerned that the story, script, and gameplay all fit together. Making sure that all of the documents are consistent with one another and are in line with the vision and focus of the project is something the designer needs to take very seriously.