What's a Game Design Document (or GDD)?
Why is a GDD helpful?
A GDD is helpful because it helps you out with two things.
1) It reminds you of what the game is supposed to be.
Without a GDD, it can be difficult to keep straight exactly what the original idea behind your game was. For example, without a GDD, you may not be able to remember how many zones there were supposed to be in the game, or what the general size and structure of the game was like. The other thing a GDD does is:
2) Keep you from adding things that don't line up with that original vision.
It's easy to get distracted while developing a longer-term title, and find that you stumbled upon yet another cool, new gameplay element, or an idea for another level that works better than the ones you have currently. The GDD helps you to stick to the ideas you have down already, rather than chase new features and content. Now, there's nothing wrong with finding a better way to implement a feature, or even to replace major sections of the game based on ideas and the general feel of the gameplay. However, the GDD should reflect the current goal of the game. If you have ideas and "updates" that you want to apply to the game, then you should update the GDD accordingly, so that you don't get distracted, and can really focus on pre-determined aspects of the game.
The short of it is, I'm personally finding it a lot easier to work on Valchion when I have a GDD to guide me. With the document, I can get an idea of which zones lead to which other ones, and what pre-requisites there are to access zones (i.e. if you need a certain item). This helps to make the flow between areas easier to understand and build the game world around.