SentenceKnown topicNew information
زَيْدٌ أَخُوكَZayd as an identified personHe is your brother
أَخُوكَ زَيْدٌYour brother as the topicThat brother is Zayd
The first sentence suits a hearer who knows Zayd but not his family relationship. The second suits a hearer who knows the brother as a role but does not know that the brother is Zayd.
This is more than a mechanical exchange of word order: the implicit question and its answer have changed. It illustrates how the subject-predicate organization serves the communicative situation.