Knowing (with fair warning) that something is going to be late lets me set the right expectations. With a month to go I can usually persuade a customer to slip a week because something just needs needs more time. I can also reconsider my time estimates for future jobs to make sure that in future the engineers get the time they need.
What I can't do is keep promising delivery "tomorrow" to external people, because I just found out today that you won't be done on time. That's bad for everyone.