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).