Steering knobs make it easier to turn the steering wheelchair using one hand, but steering knobs do require that the driver be able to grip the knob as it is difficult to maneuver using only the palm of your hand. The knobs are attached to the steering wheel to give the driver more comfort and ease when making turns or controlling the wheel.
It's possible that the product is relatively new or not well-known, or that there simply hasn't been a review posted yet.