The purpose of the USATF course certification program is to produce road race courses of consistently accurate measured distances. The certification program is very important to the average road racer as well as those of exceptional speed. Most runners like to compare performance run on different courses, and such comparisons are difficult if course distances are not reliable. No one can truly establish a personal best if the course distance is not accurate.
The USATF defines a race course as the shortest possible route that a runner could take and not be disqualified. A runner might not follow the shortest possible route, just like a runner on a track may be forced to run further to pass another runner. The shortest possible route is a reasonably well-defined and unambiguous route that ensures all runners will run at least the stated race distance. Even if you are racing on a course that is not certified, you can still implement the technique of using the shortest possible route to your advantage.