وَأَمَّا بَيَانُهُ أَيْ تَعْقِيبُ الْمُسْنَدِ إِلَيْهِ بِعَطْفِ الْبَيَانِ فَلِإِيضَاحِهِ
As for explaining the subject by following it with ʿaṭf al-bayān, its purpose is clarification.
ʿAṭf al-bayān is a dependent expression that specifies the meaning or reference of an earlier noun without joining a second subject by a coordinating conjunction. It expresses the same intended referent more clearly.
This is useful where more than one possible person or interpretation is available. The appositional expression helps the hearer identify which one is meant.