Boomerang Shackles?

Does anyone know what the benefit of boomerang shackles is?
Just curious,

 

They're intened to keep the leaf springs from "inverting" the shackle against the frame. When I say inverted, I mean the spring gets unloaded when the wheel/axle drops away from the frame when flexing and the shackle flips over and lies flat against the frame. When the tire is back on the ground and the spring is loaded again, it can't return to its normal position, severely limiting spring movement and making the Jeep drive really funny. I think they're mostly used on YJ's. Clear as mud?

 

They are for the rear of YJs for more travel.
And what mruta said.