There is a lot of waste in the average fleece. There is all the vegetable matter (VGM) that is inevitably collected as the sheep lies down in the sun or squeezes through a hedge. There are the mucky bits around the the tail, and the felted bits around the belly, none of which is useful for anything but compost.
We want to pay a fair price for our wool, and we want to get the best return on our investment. Fleece is sold by weight, as well as by fineness, so the amount of waste wool is an important factor when we examine them. If the sheep have been shorn on straw or fed from overhead hayracks the fleeces will collect lots of extra VGM. This adds to weight, but more importantly adds to the waste as more of the fleece will be rejected at the mill, so we get less yarn back. Let's not spend too long thinking about what we'd be paying for if fleeces haven't been skirted (had the icky bits removed).
To make sure we get the quality of fleece we're after we inspect the wool before we buy. We look through the wool sacks, pull fleeces out and examine for quality. We might know what a specific fleece is meant to be like, but sheep don't always understand what is expected of them, so fleeces can be remarkable off point. Shearing is another variable, bad shearing gives rise to second cuts (where the shearer has not cut the fleece close enough to the skin on the first pass, so goes over it again) which reduce the staple length and result in waste at the carding stage.
There have been a few bemused sheep farmers who have watched us go into raptures over particularly good fleeces, and have suddenly understood how much good wool is to be valued.