Consider three expressions:
«حَضَرَ خَالِدٌ» — Khālid arrived.
«حَضَرَ الرَّجُلُ» — The known man arrived.
«حَضَرَ أَخُو خَالِدٍ» — Khālid's brother arrived.
The first identifies through a proper name. The second, where «ال» refers to a previously established person, uses prior-reference definiteness. The third identifies through annexation.
Crucial distinction: all three might in some context concern the same actual person, but the linguistic path to that person differs. Rhetoric evaluates which path is appropriate. If the hearer knows someone chiefly as ‘Khālid's brother,’ an annexed expression may identify him more effectively than a name.