To be clear, relationship designators are not core/required by RDA. However, the Program for Cooperative Cataloging strongly encourages using them when you know what the relationship is.
The relationship designators are intended to make the authorized access points more like the RDF triples of the Semanitc Web. We have to make these relationships explicit for machine manipulation, otherwise the machines won't be able to parse out what the relationships are between entities. It helps to think of the access points as discrete data elements that can be utilized by search software, rather than a part of a catalog record being read by a human in its entity.