For which condition are parking brakes primarily intended?

Parking brakes are primarily intended to secure the vehicle when it is parked. This is crucial because it prevents the vehicle from rolling away unintentionally, which could lead to accidents or damage. The design of parking brakes, often referred to as emergency brakes, allows them to hold the vehicle in place even when it is on an incline or slope.

The other options address different aspects of vehicle operation that are not the primary purpose of parking brakes. For instance, assisting in acceleration, enhancing braking power during driving, or controlling downhill speed are functions related to the vehicle's service brakes and overall driving dynamics rather than the specific function of parking brakes. The focus of parking brakes is solely on maintaining the vehicle's position when it is stationary.