Why are new properties required for an unbound node type?

I'm hoping someone smarter than me in the community can help me understand the following:

SITUATION: I'm rebuilding a dimension from end to end in a development pod. Our original build used a single node type for each application from Inbound (where we onboard source data) through a Master (Planning application) and on to the targets (FCCS and EPBCS). My rebuild is intended to introduce multiple node types to distinguish parent, base, and special purpose nodes in each and every application from the Master through to the targets. On that journey, we have a mapping viewpoint that uses the node types of the Master for map targets and an unbound node