In evolutionary classification, knowing how an organism is classified is insufficient for either purpose. The principle of divergence explains why phylogeny is hierarchical. Phylogeny itself is part of the answer. But if descendents remained too much like their ancestors, all descendent groups would overlap. And if descendents departed too dramatically from their ancestors, there would be no similarity within groups.
Gradual divergence offers an explanation.
The fact that evolutionary change is necessarily gradual assures similarity among descendents of a common ancestor. Here Ridley summarizes the strengths and weakness of the three principle schools of classification. You don't need to choose among them, but you should be aware of how they differ.