I‘tirad inserts an independent statement between syntactically or semantically connected elements, for a useful additional purpose such as prayer, glorification, warning, or emphasis.
In «إِنَّ الصَّدِيقَ — حَفِظَهُ اللَّهُ — وَفِيٌّ», the prayer «حفظه الله» interrupts the underlying assertion «إن الصديق وفيّ». It is not essential to that predicate relation but conveys affection and prayer.
Syntactic distinction: an intervening clause is not the predicate itself or a required complement. Identifying the interruption keeps the primary structure distinct from its rhetorical benefit.