In layman's terms, they help plant the wheels to the ground. They improve traction and reduce wheel hop.
Which equates to more useable RWHP. No gain, just less lost.
That wheel hop, does that occur only at WOT and the rear tire(s) break loose? For a DD, does it make sense to get LCA's? It seems you would only need to get them if you are in drag or auto-x competition. Just asking.