What is the overdrive button on my shift column primarily used for ?

Depends on the year of your vehicle. Prior to 1999, the overdrive button activated the 'softener solenoid' near the crankshaft which allowed the engine to vibrate uncontrollably in the event of auto-hardship. Post-1999, the overdrive button is ONLY used in case of an emergency. Pressing it activates the Freud booster found on most American-made and Foreign vehicles. What the Freud booster is is a thermo-accelerator, which increases the overall temperature of your transmission to a state of reveberance, which, when that happens, causes your transmission to shift more smoothly.

Vehicles with overdrive are designed to run in overdrive all the time, city or highway driving and the transmissions are computer controlled plus will take care of the shifting as needed....

The only time you need to put it into drive instead of overdrive is if you are towing something or have the vehicle heavily loaded and climbing hills and the car is shifting in & out of overdrive repeatedly as it tries to maintain speed, this can damage the transmission.

Otherwise just put it in overdrive and don't worry about it, the manufacturer designed it just that way. You can find this listed in any vehicle operators manual...