Deadlines feel like they are mainly useful to induce stress and urgency. The stress and urgency being useful (or important) is another matter.
In a perfect world (for me, from my point of view), there are no deadlines, but rather short plans and enough understanding of the plan to decide whether doing something specific is useful or a hindrance (and to what degree) for executing that plan.
A genuine deadline is good, an artificial one isn't. Sometimes budget limitation can be a genuine hard deadline. A fixed special days, such as new year or election date, can also make a hard deadline. That being said, you'll need to move the deadline some time ahead to provide buffer.
Shorter, soft deadlines and milestones are also good, since they provide a measurement between long term planning and current progress. Without them larger projects will likely failed than success.
However management loves short, hard deadlines with artificial reasons, which makes them stressful.
Your comment made me realize my problem with deadlines. Whenever I hear about a deadline, it has always been the case that management would like to have a time constraint without giving in from any of the other constraints, e.g., scope, budget, quality. It's like trying to find a solution to a set of equations when constraints guarantee that there is none.
Deadlines are great for keeping scope in check. Almost any feature can be developed with endless embellishments - without some kind of deadline it's not clear how you can decide how much of those embellishments you should add before moving on to the next feature.
In a perfect world (for me, from my point of view), there are no deadlines, but rather short plans and enough understanding of the plan to decide whether doing something specific is useful or a hindrance (and to what degree) for executing that plan.
But alas, deadlines are easier to decide on.