I think we can all agree that few things motivate you to actually open up the word doc and WORK on it as well as a deadline. Whether it's real--you have to deliver work to an editor, or even just pages to your critique group--or totally self-imposed, knowing the "must have by" date is a very effective lighter fluid for the all-important fire-under-the-ass. It gives you a clear sense of how to structure time; knowing where the finish line is lets you work backward, and parcel out your to-do list into little time-packets of however much work you need to get done, in whatever way works best for you.
I may be the only person who thinks of it that way, but then I'm totally OCD.
I realize that the "whatever way works best for you" part is open to serious interpretation, of course. My college roommate rarely started a paper more than 24 hours before it was due. She usually wound up working til some time in the very-early hours of the morning, fueled by a near-constant stream of coffee and cigarettes, but she always got it done, and from what I heard, she usually did a kickass job. She needed to be in a serious pressure-cooker situation before she could be bothered to really focus on the problem at hand.