This "arbitrary fields" example is related to the idea of custom blocks, where certain use cases will require custom data types embedded into the document hierarchy.

Sometimes an account will want to publish data about some other resource that it might not have access to write to. Like leaving a comment with some helpful metadata.

In some cases a document owner may want to acknowledge an attribute blob as "official" metadata, saving it into the history of the document. Rather than saving directly into document metadata, linking to the attribute blob would preserve the original author and timestamp