Overdrive simply give you another gear, which used to be fourth, then became fifth, and now on the newest cars is 6th (new navigator has a six speed automatic) You don't really hear it called overdrive in manuals, however generally its the last gear. 5th in the mustang, 6th in the vett, etc... All it realistically does is allow you to go faster at a lower RPM, conserving gas mileage. Can you think of any other reason to put six speeds in a 7200 pound lincoln? It also aids in emissions somehow, I don't remember exactly but the new mercedes S430 and S500 utilize a six speed, which has really ruined the cars performance, to meet emissions standards for 2006
One known use is for towing, especially in very steep grades. Notice how when you drive up a hill, slightly graded, that the car will shift up and down and up and down, never finding its happy place? Engauging the overide for autodrive drops it down a gear and "locks out" overdrive, allowing you to operate without constant shifting