They elevate the front of the truck while the rear gets lowered or left alone. The front end of the truck is higher than the rear so that when you hit a jump at a high speed the rear hits first. It reduces the chances that your truck will nose dive and crash when hitting hills fast and going airborne.
Granted, some people just like the look of such a truck set up. Some Cali Lean configurations are subtle compared to others. Outside of aesthetics, are there any practical reasons for this trend?