When I was studying power generation and distribution back when, I was taught that the Earth (and little areas of earth) were simultaneously infinite sources and infinite sinks of electrons. The instructor illustrated this by talking about a generator separated by some miles from a load, one hot conductor connects the two and the return is the earth (a single wire on the pole). At high generation voltage and low utilization current the earth's impedance becomes negligible (think of a 30 A 120/240 V load on a stepdown xformer from an 8 KV xmission line - less than 0.9 A will be called for). It was quickly discovered that the area of earth around the load would develop a different charge than the area of earth around the generator, essentially static charges that would concentrate and dissipate with respect to natural phenomena, most notably, lightning. So the earth, at any given instant, becomes a second voltage source that can either add to or subtract from the generator, sometimes with deadly results.
Running a second conductor on the xmission line that was grounded at intervals does several things. When placed above the 8 KV hot conductor, it shields the hot conductor from lightning strike. This grounded conductor also Short Circuits the earth's charges from one ground connection to the next. A concentration of electrons in one area of earth will flow easily to an area of absence. The end result is, that in the immediate vicinity of the xmission line, the potential of the earth itself becomes more uniform along the length of the xmission line and the voltage at the load end is stabilized.