On the other hand, setting a deadline can't force something to be possible, it can only force people to work harder and more painfully towards it. I'm sure the Amazon drone delivery failure had a date target, for example. And there have been plenty of failed "big bang" IT migrations delivered by similar immovable deadlines.
It's so you can (hopefully) have an educated understanding of setting that deadline, and then how you're progressing towards it. It's so if you're not on track to deliver, you can make more educated decisions about cutting scope, or adding resources (yes yes yes, i know).
The desired features beyond the minimum set are accepted as being at risk of remaining unimplemented if the due date can't be shifted further to the right. They become stretch goals that may be achieved by the deadline, or will be worked into a second effort during the maintenance of the system.
Agile (especially some of the stuff in the Lean Software world) is very well-suited to this kind of development. As are the classical evolutionary or iterative & incremental models.