It is the mark of an educated mind to rest satisfied with the degree of precision which the nature of the subject admits and not to seek exactness where only an approximation is possible —Aristotle (384 B.C - 322 B.C.)
When we hear someone say estimates are guesses, When we estimate we act as if we believe the plan will not change, or similar uninformed nonsense, think of Aristotle. Without the understanding, from education, experience, and skill to realize that all project variables are random variables and vary naturally and vary from external events.
As such in order to determine the future impacts from decisions that involve cost, schedule, and performance, we need to estimate that impact of those random processes on the outcome of our decision.
This is the basis of all decision making in the presence of uncertainty. It's been claimed decisions can be made without estimates, but until someone comes up with the way to make decisions without estimating those impacts, statistical estimating is the way.