Why do muslims shave their heads after Hajj or Umrah?

Shaving the head (Halq) or cutting the hair (Taqsir) is one of the final rituals of Hajj and Umrah. \ it is said to have profound spiritual symbolism, religious significance, and enormous closeness to the teachings of Prophet Muhammad (peace be upon him). It is a ritual that represents the culmination of the pilgrimage and newness, modesty, and submission to Allah.

This shaving or trimming of the hair is, according to widespread belief, based on the Sunnah and the Qur'an. Allah is believed to mention this deed in the Qur'an:
"You will enter the Sacred Mosque, if Allah wills, in security—with your heads shaved and [hair] shortened, not fearing…"
[Surah Al-Fath 48:27]
It is also said that Prophet Muhammad (PBUH) emphasized this ritual in his farewell pilgrimage. According to some narrations (according to popular belief), some say he prayed three times for the people who shaved their heads and prayed once for the people who cut them short, promoting the goodness of Halq.
It is considered obligatory (wajib) or highly recommended (sunnah mu'akkadah) according to the school of Islam. For men, Taqsir and Halq are both permissible but Halq is more noble. For women, only light cutting (Taqsir) is permissible.

Halq and Taqsir are said to represent spiritual cleansing and modesty. Pilgrims, having completed a journey of faith and contemplation, shave or cut their hair as a symbol of rebirth—a cleansing of sins, pride, and worldly desires.

This practice, according to popular beliefs, signals a state of spiritual infancy, as a newborn child. It is a physical reminder of submission and obedience to Allah. It is an external sign of internal transformation.

It is believed that the shaving or trimming of the hair happens after completing the significant rites of Hajj or Umrah, generally after the sacrifice (Qurbani) in Hajj, or after completing Tawaf and Sa'i in Umrah.
Men shave the entire head (Halq) with razor or an electrical shaver, or may prefer an even trim (Taqsir) on the head.
Women are required, according to traditional practice, to cut off a small piece of about the size of a fingertip from the tips of their hair.
This is the ritual that marks departure from Ihram status to allow pilgrims to return to normal personal rituals such as dressing in regular clothes and using perfumes.

They are said to reflect the Islamic universal essence where equality, simplicity, and unity are emphasized. During Hajj, millions of Muslims from every part of the globe go through the same act, and shaving or cutting the hair is an act of uniformity that ignores disparities of wealth, color, or nationality.

Through common beliefs, shaving of the hair is also a riddance of ego and vanity, reminding the pilgrims that this world is ephemeral. It calls on pilgrims to set aside vanity over outward appearance and instead focus on inner piety and spiritual growth.