Mentioning the wisdom behind this, Mufti Aḥmad Yār Khān NaꜤīmī رَحْمَةُ الـلّٰـهِ عَلَيْه states:

The Prophet صَلَّى الـلّٰـهُ عَلَيْهِ وَاٰلِهٖ وَسَلَّم   was sleeping in the home of Sayyidatunā Umm Hāniˈ bint Abī Ṭālib رَضِیَ اللهُ عَنْهَا when the angels woke him and took him to the Ḥaṭīm of the Kaaba. At this point, he صَلَّى الـلّٰـهُ عَلَيْهِ وَاٰلِهٖ وَسَلَّم was still overcome with tiredness. He صَلَّى الـلّٰـهُ عَلَيْهِ وَاٰلِهٖ وَسَلَّم   was then bathed. In the case of an ordinary bridegroom, just the body is washed, but the Prophet صَلَّى الـلّٰـهُ عَلَيْهِ وَاٰلِهٖ وَسَلَّم   is that eminent person whose heart was also washed.

The water of Zam Zam is superior to other waters because it flowed due to the foot of Sayyidunā Ismāˈīl عَـلَيْـهِ الـسَّـلَام, which is why it was used. The heart of the Prophet صَلَّى الـلّٰـهُ عَلَيْهِ وَاٰلِهٖ وَسَلَّم   was filled with faith and wisdom and then sewn together. Faith and wisdom were already present in the heart of the Prophet صَلَّى الـلّٰـهُ عَلَيْهِ وَاٰلِهٖ وَسَلَّم, so this was only to augment them.^[1]

The pure chest (of the faultless Prophet صَلَّى الـلّٰـهُ عَلَيْهِ وَاٰلِهٖ وَسَلَّم) was already filled with light, so now it became even more illuminated. The gold was heavenly, and the water was Zam Zam; the water of the Ḥaram in a golden vessel of Paradise. سُـبْحٰـنَ الـلّٰــه! This added to the beauty!^[2]

The wisdom of opening the chest

It is written on pages 79 and 80 of the book Sīrat-e-Muṣṭafā, a publication of Maktaba-tul-Madinah: