They help to preload the rearend, which helps the tires hook much better, especially when power brake launching with a lot of boost.
Also, they are very helpful in reducing sway that comes when you remove or disconnect the front sway bar for the most weight transfer. They also help keep large tires from rubbing the wheel trim.
If you never intend to take the car to the drag strip, or remove the front sway bar, they aren't really needed.
HTH
Patrick