Gears allow you to travel faster without the engine overheating. They are simply levers arranged around wheels of different sizes. See these "gear cogs" arranged inside a "manual gearbox".
A gear-change is made at (1) the right speed, (2) when your left hand is free, (3) if there's little activity outside, and (4) at a good pace. Here's what happens when you move the "gearstick".
You don't need to be in a certain gear to deal with a certain road feature. Your CONTEXT defines your safe SPEED, which in turn defines the best GEAR. But you'll need to use the clutch pedal.