Think about teaching your kids peogramming. Do you set deadlines on when they need to finish learning to read, when they'll touch their first computer, the date they need to have a working hello world ?
For any of these you'll probably be buying toys, books, discuss with your partner and arrange time etc. But none of those will be "deadlines", even if you intend to be very optimal in the progress
Kids learning something is quite the opposite to conplex projects out in the professional world. Coordination without schedules won't work, and schedules include deadlines for certain milestones.
I think it's pretty similar, and what many think of as deadlines aren't, in the sense that they are flexible.
For instance infrastructure projects: a delivery date is set, but developpers blowing out that date is just a fact of life. What will you do anyway when your new dam or highway is 6 months late ?
Even at smaller scales, you'd schedule moving to a newly built house at a set date, but painfully know the developper can easily blow past that and you can't just take the date for granted, or you accept moving to an unfinished house.
Software projects are of course the same, at big and smaller scales. Just look at game delivery for the most piblic instances of setting "deadlines".