You've done it before — knife in one hand, slab of meat sliding around on the board, trying to shave off just enough fat without carving into the good stuff. It takes forever, your grip gets greasy halfway through, and one wrong angle means either wasted meat or a nicked knuckle. So the untrimmed cut goes back in the fridge, or you pay extra for the pre-trimmed one at the counter instead.
A butcher's trim costs more than the cut itself, and a knife class is overkill for something you just want handled before the smoker's hot.