After seeing a picture, i realise this is not what i had pictured.
Question.
Is this done mostly to get the car lower to the ground or is this a traction aid???
The lowering is pretty minimal, I wouldn't install them just to lower the car. The reason it's done is that they improve spring reaction, reduce binding, and have less lateral flex than a set of shackles. They allow the leafs to move more freely and in a direction that better matches the spring action (shackles force the spring eye to travel in an arc).