A tractor is asked to perform very different jobs: ploughing through heavy soil, hauling loaded trailers on roads, and driving stationary implements through the power take-off.

Each of these tasks demands a different balance between travelling speed and the pulling force (torque) available at the wheels or drawbar.

A manual transmission provides a range of gear ratios, letting the operator select a low gear for maximum pull at low speed or a high gear for faster travel under light load.

By changing gears the driver adapts the tractor to whatever the field or road conditions demand, keeping the engine working in its efficient speed band throughout.

A fixed gear system would lock the machine to one ratio, so it could not trade speed for torque and would stall in heavy work or crawl on the road.

The other options are wrong in content: gears do not force a constant speed (option 1), they are unrelated to the braking system (option 2), and their purpose is power delivery, not noise reduction (option 4).

Hence, the answer is They allow tractors to adapt to varying work conditions.