The only times you might consider manually selecting a lower gear (if your car even allows it – many automatics don't offer this level of manual control) are:
* Engine Braking on Steep Downhills: In very steep downhill situations, manually selecting a lower gear (like second or even first, depending on the steepness and your car's capabilities) can help slow the car down, reducing reliance solely on your brakes and preventing brake fade. However, this is generally a last resort or for vehicles without sufficient engine braking in a lower automatic gear.
* Extreme Off-Roading (rare): In very challenging off-road situations where precise engine control is crucial for traction, you *might* find a need to select a lower gear manually. But even then, it's usually a specialized case and not standard driving practice.
In almost all everyday driving scenarios, leave the gear selection to the automatic transmission. It's designed to handle this far better than you can. Using lower gears unnecessarily can cause increased wear and tear on the transmission and engine, and reduce fuel economy.
