فَلِتَخْصِيصِهِ بِالْمُسْنَدِ
Thus, its purpose is to specify the musnad ilayh through the predicate.
The central concern is the predicative relation. A predicate is not simply asserted in general; its attribution is presented as particular to the subject. In «Zayd huwa al-qāʾim», standing is associated exclusively with Zayd in the intended rhetorical reading.
This can be useful when a hearer knows somebody is standing but needs to identify the person intended.