The only certainty is uncertainty - Pliny the Elder (Gauus Plinus Secundus) (Natural History)
When the cost of a future state is considered the decision makers ask What is the chance the system's cost will exceed a particular amount? Or what are the uncertainties and how do they drive cost? Cost, schedule, and technical uncertainty analysis provides the decision makers insight into these and other important questions.
These uncertainties come from inaccuracies in cost and schedule estimates. They come from the misuse, misrepresentation, or misinterpretation of estimating data, or misapplied estimating methods. They come from intentionally ignoring the probabilistic and statistical nature of all project work.
But these uncertainties do not remove the need for the decision maker to know about the probabilities associated with the project, to some level of confidence of the cost, schedule, technical performance, and probability of project success.
This knowledge is needed before and during the project. Without this knowledge the very notion of making decisions is uninformed by the raw data needed to decide.
To not know, not be able to know, or not want to know, means basing decisions on ignorance of the emerging situation of the project. And since all project management processes are about making decisions, to make those decisions we need credible information. Estimates are one of those decision making pieces of data. To not have an estimate is to intenntionally ignore a piece of information critical to the sucess of any project.
So before you listen to anyone suggesting we don't need to estimate cost, schedule, and technical performance, ask them to show you their marked up copy of the book below (one of several dozen hands on guide books for estimating project cost and schedule) or have them point to where not knowing the cost, schedule, risk, or technical performance to some degree of confidence, before starting the project or during the project, is in the best interest of those funding that project.