Grounding electrodes

The short answer to this question is simple: money! Why have an engineer do blueprints and drawings for your building? Because you need to have a plan before you start construction. Could you imagine a construction company showing up at a site without blueprints and not knowing if they were going to build a skyscraper or an outhouse? In order to achieve the 5-ohms resistance-to-ground specification required for many sites, the grounding system could be as simple as a few driven rods to as complex as 40-foot deep electrolytic electrodes with huge radials running hundreds of feet away from the site.
Digging up the earth is one of the most expensive things on a job site. Engineering it in advance ensures efficient use of construction time and ensures that you will hit the desired resistance targets.