The rods come in different sizes in order to facilitate the variety of needed installations. Eight foot rods are the minimum length that the code allows but the specifications on the job may call for an identified impedance to ground that can only be achieved with stacked rods or rod arrays arranged over a wide area. When rods are to be stacked or coupled to achieve a greater depth it is obviously less expensive to use ten foot rods coupled together than it would be to use eight foot rods. Extremely low ground impedance values can often be achieved only with deep driven rods of forty or more feet in length.
"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison