@andrew_bowman has a great example when such a dependent entity (a report card node) should always be created with a relationship to its parent (a student node). But you can later delete its relationship, and leave a report unrelated to anyone.
Relational databases emulate dependent entities with PKs including the PK of their parent. And you can make sure, with CASCADE DELETE/UPDATE, no child will remain without its parent.
I'm only curious if there is anything in Neo4j to prevent deleting the last relationship to such a node emulating a dependent entity.