One might object that if both subject and predicate are definite, nothing new is being said. The answer is that knowing two referents and knowing their relation are different kinds of knowledge.
A listener may know Zayd but not know that he is ʿAmr’s brother, or may know of a departing person without identifying that person as ʿAmr. A definite predicate can reveal the previously unknown relationship.
However, if the listener already knows the identity and the speaker has no further rhetorical aim, repeating it may add no real information. The hearer’s state of knowledge is therefore decisive in judging the usefulness of the wording.