I know in the other thread I said it was there to get your vision across to others, but I wasn't completely correct on that point...

It is also there to help keep you focused on what you envisioned the end product to be. In my mind, it is kind of a reverse sculpting... you start with the central idea, and add a little bit here, a little bit there, until the end looks like what you had in mind from the start.

You don't have to wait to develop the game until you've completed the document, but in it's way, it is like having the directions to your destination before you turn the key in your car. Yeah, if you know the general direction you need to go, you will eventually get there, but it may take you twice, three or ten times the amount of time it would have taken you if you had planned the route in advance.

This does not mean that if you plan, there won't be roadblocks along the way, but you'll be better prepared to deal with them and able to get back on track sooner if you had the plan in the first place.