Good question. Perhaps my phrasing has left room for interpretation. What I meant to convey is that the choice of the dubbing, and the amount of twist, allows the tier a measure of control over the final look&feel. The full gamut from very coarse looking result to tight, segmented, bodies.
Another consideration is how short staple dubbing is handled. Some tiers struggle to twist short staple dubbing, for example mole, around the thread into a tight noodle. Using split thread this material handles very easily. Similar with very short staple synthetics.