Not just done, also the end date. If your customers have a specific date where things must be done you dare not lose track of the end date. In some fields congress has said things by some date, and things are bad if you fail to meet that date. If you are targeting mars, then if you are not ready for launch month your next opportunity is over a year latter. On large projects all the parts need to come together at the same time, if software is going to be late the mechanical engineers get more time.