These are still most commonly seen in continental European countries such as Germany, but are becoming more popular in the UK.
With a Swan neck tow bar instead of the neck of the tow bar ending in a flange plate (to which a separate tow ball is bolted), the neck turns straight into the ball. Many people believe that they are more aesthetically pleasing than the more common flange type, as usually the electrics are located on the main tow bar out of normal view.
However they do have disadvantages in that traditional blade type stabilisers (Bulldog, Snakemaster etc) often require an expensive and intrusive adapter. Also bumper protector plates, tow steps and pins and balls etc cannot be fitted.
The design has developed itself in close collaboration with all the major car manufacturers. It is important to keep in mind that the car manufacturers' calibrations are all based on the "swan neck" design.