They are used to rotate the overall toe of the rear axle. Because its a solid axle, you can't adjust each wheel's toe individually, but playing with the length of each adjustable trailing arm lets you rotate the axle's toe, so ideally you want the axle centered with equal toe to each wheel. I know when Whiteline used White Knight as the test project car, it originally had pretty uneven toe to each rear wheel. So they developed this to even it up.
I totally agree, you can't really notice it in real life and its mostly an ideal theoretical thing to correct. But it can improve tyre wear a bit if say one wheel is dragging a lot more due to uneven toe than the other wheel etc.
I wouldn't go out and buy another set if I had to start from scratch again.