وَهُوَ تَأْدِيَةُ الْمَعْنَى بِعِبَارَةٍ زَائِدَةٍ عَنْهُ مَعَ الْفَائِدَةِ
Itnab expresses a meaning in fuller wording that contributes a benefit.
قَالَ رَبِّ إِنِّي وَهَنَ الْعَظْمُ مِنِّي وَاشْتَعَلَ الرَّأْسُ شَيْبًا
He prayed: My Lord, my bones have weakened and my head has flared with grey hair. (Maryam 19:4)
“I have grown old” would communicate the bare fact. The imagery of weakened bones and hair spreading across the head conveys frailty and earnest supplication more vividly. The enlargement therefore has a genuine rhetorical function.