This is typically used for:
* Towing or hauling heavy loads: Lower gears provide more engine braking and power at lower speeds.
* Steeper inclines or mountainous terrain: More power is needed to maintain speed.
* Better engine braking on descents: Lower gears help slow the vehicle without relying solely on the brakes.
Essentially, it limits the highest gear your transmission will use, making it more powerful at lower speeds but potentially reducing fuel efficiency at higher speeds. You'll want to turn it back *on* ("OD ON") once you no longer need the extra power or engine braking.