«وَأَمَّا تَعْرِيفُهُ فَلِإِفَادَةِ السَّامِعِ حُكْمًا عَلَى أَمْرٍ مَعْلُومٍ لَهُ بِإِحْدَى طُرُقِ التَّعْرِيفِ»
Making the predicate definite can inform the hearer about the identification or relation of something already known, by one of the recognized ways of definiteness.
Knowing two people or descriptions separately is not the same as knowing that they identify the same person. A hearer can recognize Zayd and independently know of someone’s brother, while not knowing that the brother is Zayd.
A definite predicate can supply this missing relationship or identity. But which expression is made the grammatical subject depends on what is already fixed as the topic for that listener.