In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

Imaam As-Sadi [may Allaah have mercy upon him] said:

What the Sharee’ah brought regarding the transfer of wealth and inheritance after death, and how to distribute wealth to the heirs: the Exalted [Allaah] has pointed to the wisdom of this, saying: [لا تدرون أيهم أقرب لكم نفعا – You know not which of them, whether your parents or your children, are nearest to you in benefit, (these fixed shares) are ordained by Allah]. [Surah An-Nisaa. Aayah 11]

So Allaah himself fixed these shares by way of what He knows perfectly – (and free is Allaah from all imperfections, weakness, faults etc) – regarding what is closest to benefit, what a person usually loves with regards to who his wealth should reach and who is more deserving of his kind treatment and favour, and arranged it in an order that sound minds would testify to its excellence. And if the matter were to be entrusted to people’s opinions, desires and wants, it would have resulted in defects, disorder, bad choices and the disappearance of order that would be tantamount to chaos.

Allaah commanded a person to direct some of his wealth in causes of righteousness and piety that will be beneficial for his Hereafter, and restricted that to a third or at least lesser than that of a heir, lest affairs which Allaah has secured for the people become a personal playhouse – at the time of departure from this world – for those people who are deficient in intellect and religion. As for when they are in a state of good health and stable minds, then in most cases what they fear of poverty and bankruptcy prevents them from spending wealth on what harms them. (1) [End of quote]

In order to realize the beauty of this subject matter and its details, then must listen to lessons on this link https://www.salafisounds.com/playlist-ar-rahbiyah-principles-in-the-laws-of-inheritance-by-abu-hakeem-bilal-davis/ by Shaikh Abu Hakeem [may Allaah preserve him]. [Ar-Rahbiyah – Principles in the Laws of Inheritance]

[Ref 1: Ad-Durra Al-Mukhtasar Fee Mahaasin Deen Al-Islaamiy. pages 27-28]