What is the primary function of retarders in a vehicle?

The primary function of retarders in a vehicle is to slow the vehicle and reduce brake wear. Retarders provide an additional means of slowing down a vehicle without solely relying on the traditional brake system. This is particularly beneficial for heavy vehicles, such as trucks and buses, where braking can lead to significant wear and tear over time. By using retarders, drivers can maintain better control when descending hills and managing overall vehicle speed without overheating the brakes. This added braking power helps to extend the life of the vehicle's braking components, promoting safety and reducing maintenance costs.

The other options do not accurately describe the main purpose of retarders: they do not primarily increase speed, assist in steering, or improve fuel efficiency directly, although better braking control can lead to more efficient driving practices overall.