Adjustable rockers on hydraulic lifters allow you to taylor the lifter preload to what you want out of the engine. You can actually change how the engine operates to a degree by changing preload. For instance, I like preload light. Usually around .020 into the plunger travel. Sometimes this can result in a little valvetrain noise....most of the time not. What it does do is allow a few more RPM out of the engine. The effect is not as pronounced as tayloring solid lifter lash, but you can make a small difference. The biggest thing to me is simply making sure there's not too much preload from things like milled heads, decked block and or thin head gaskets. Lastly, the iron rockers are MUCH stronger than the stock paddle type. Much more than stock spring pressure and you risk punching through the pushrod cup in the rocker. The iron rockers eliminate that issue altogether.
