Dyno run parts

I was cruising some classifieds and occasionally I run across parts that where only run during dyno time. For instance today it was a set of lifters and a bronze bushing intermediate shaft. Im not looking to buy these parts, but I dont get something either. Im just trying to wrap my head around why these parts are swapped out so fast.

Why do a couple pulls on a dyno and then swap out the parts with something else? Is it that I just dont have the money to play like some of these people do? Could someone please explain to me why this is done.