Preload is basically how much pressure is put on the lifter when fully closed, to tight and the valve won't close fully and too loose and you will hear ticking sounds. With valve train alignemnt, you want to align the rocker height so it pushes down on the valve in the center of the valve stem so you don't get any side loading on the valve.

My understanding is that with adjustable rockers, you can fine tune the preload and alignment with the adjustment screw on the rocker arm while with non-adjustable ones you use shims or use different length pushrods. I have only used non-adjustables in the past and have never had any issue getting good preload and alignment.

Ahhh gotcha. I always wanted to know what the benefites were. Thanks.