Rear shocks to rear of wishbones etc
 

So sceral cars have this option now days (Trf511, Cat Sx), or run this way all the time such as CR2, the X factory cars etc, but wondered what the idea behind moving the rear shocks to behind the wishbones is compared to running them in front?

I've heard it's good for bumpy tracks but why? What are the techy reasons for/against? For those who have tried both on a car, what are the practical effects?