DHH:

The purpose of a self-imposed deadline is to sharpen the edge of your prioritization sword and stake a flag of coordination for the team. It’s not a hill to die on. It’s not a justification for weeks of death marching. It’s a voluntary constraint on scope.

Yes, deadlines are wonderful! They’re the tie-breaker on feature debates. They suck all the excess heat out of the prioritization joust: “Hey, I’d love to get your additional pet feature into the first release, but, you know: THE DEADLINE”.