Excerpt from The Writing World Defined--A to Z (order at www.stuartmarket.com)
Deadline. The final date, specified in your contract or correspondence with an editor, for turning in your manuscript. For a writer, there are few things more important for building their career than meeting those deadlines. Your deadline is set as part of the editor's production schedule, and if you miss the deadline, it throws off the schedule of all the other people involved in the production of the magazine or book. If the deadline is with a periodical, it could mean your piece would be dropped from the scheduled issue and reassigned to a future one, or you could simply be replaced and you would lose the sale altogether. The worst-case scenario would be that the publication would not buy anything from you in the future.
The ramifications of missing a book deadline could be even more dire. There are a lot of people and services involved with putting out a book, and all those services are scheduled ahead of time based on your deadline. If you miss it, you basically move to the end of the line and all the services have to be rescheduled. Even worse from your perspective is that the editor may lose interest in your project and move on to working with other authors—those who meet or beat their deadline.
Of course, there are sometimes legitimate reasons for missing a deadline, such as extended illness or a death in the family. If something like that should come up and it looks like you are going to miss the deadline, let the editor know immediately—do not wait until the deadline is there or passed. The sooner you let them know, the easier it will be for them to adjust their production schedule.
Some writers actually work better if they have a deadline, so if that's the case with you, put your own writing projects on deadlines to keep you motivated. For tips on how to consistently meet your deadlines, go to: http://writeitsideways.com/how-to-meet-your-writing-deadlines-every-time-2.