A demonstrative can present an already mentioned event or person as though brought before the listener. After an antecedent has been established, using «هذا» instead of an ordinary pronoun may convey wonder, strong focus or clear identification.
هَذَا
This — a person or matter presented as vividly present in attention.
«هو» might have sufficed for reference, but the demonstrative adds a sense of presentation. “This” need not point to a physically visible object; the matter may be present only in the discourse and the listener's mind.