In a sense, this is an example of the scientific method. Before you can trust the results of an experiment, it must be repeatable. You must measure the results to know what the results are. When the experiment is measured and repeatable, then you can vary specific parameters and observe the effects. Because the process is repeatable, you know that any differences you measure come from the variation that you introduced.
Software processes work the same way.
Suppose that you want to assess the effect of using inspections rather than walkthroughs. You've used walkthroughs consistently and collected data on the error-detection rate and the effort required for each error found. If you're using a measured, repeatable process, when you substitute inspections for walkthroughs you can be reasonably certain that any differences measured are attributable to the use of inspections rather than walkthroughs. If you measure a 20 percent improvement in error detection and a 50 percent reduction in effort per error found, you can feed that result back into the process and change from walkthroughs to inspections. Summary On all but the smallest software projects, the development process you use substantially determines the quality of your programs.