Bob Wescott provides information about performance modeling and its usefulness for making predictions and assessments. He explains that in some situations, models are not only useful, but essential:
There are two kinds of important performance problems you can’t solve without modeling. You can’t do performance monitoring, capacity planning, or load testing on un-built systems, as there is nothing to test. You can’t use simple capacity planning or load testing to predict future performance on systems that are about to undergo radical transformations.