لِتَمْيِيزِهِ أَكْمَلَ تَمْيِيزٍ
In order to distinguish the subject with the fullest possible clarity.
A demonstrative directs the hearer's attention to a referent recognizable by pointing, perception, or an intelligible contextual cue. The speaker not only characterizes a person but separates that person from competing possible referents.
In «هٰذَا هُوَ الْمُعَلِّمُ», “this is the teacher,” the gesture can distinguish one person from others. If several teachers are present, successful reference depends on whether the listener can determine which person is intended. Full identification does not necessarily require more words; it requires the most effective means of distinguishing the intended subject.