ExpressionProminent rhetorical effect
زَيْدٌ قَائِمٌA plain report that Zayd is standing
زَيْدٌ هُوَ الْقَائِمُSpecial attribution of the standing to Zayd
The first sentence simply attributes standing. The second uses the separating pronoun and a definite predicate in a construction that may foreground exclusivity. Context determines which proposed alternative is being rejected.
This is not merely repetition of Zayd’s name; the manner of presenting the predication changes.