11:06 PM, Sunday February 2nd 2020
Think of it this way. You're drawing from your elbow, and you realize halfway through your stroke that you can't actually physically bend your elbow any more, but your stroke needs to keep going. So you end up engaging your shoulder to make it the rest of the way.
This is a problem because this kind of "change to the recipe" (the recipe being the ratio of how much you're moving from each pivot) results in a sudden change to the trajectory of your stroke, which messes with their fluidity and flow. This is why we focus so heavily on having students draw any smoothly flowing lines from their shoulder the whole way through. Students also have the hardest time getting comfortable with that pivot, so using it even where the elbow may suffice helps make it easier in the long run.