What is the reason for the Kaddish? Is it for the person who died, or to benefit the Mourner? TS

The Kaddish deserves an entire essay. However, I offer some basic thoughts:

Kaddish is recited for the deceased. Kaddish is recited for the mourner’s sake. Kaddish is also recited for the benefit of everyone listening and responding. There are many levels to the Kaddish.

When Jacob responded to his children, “May His great name be blessed forever,” he sanctified the name of God, Kiddush Shem Shamayim, and the angels composed the third blessing of the Silent Prayer, the blessing of Sanctity. The Aramaic translation of Jacob’s words is the “Yehai shemai rabbah,” of Kaddish. (Early sources actually refer to the Kaddish as the “Yehei shemai rabbah.” The first mention of this prayer as Kaddish is in Masechet Sofrim 16:12) All who participate in the Kaddish can fulfill the Mitzvah/Concept #5, of the Sanctification of God’s name.

When the mourner recites the phrase, ” in the world He created according to His will,” he is acknowledging God’s justice, the process known as Tziduk HaDin. The mourner is fulfilling the commandment of Honoring Parents even after their death. (Bet Yosef, Yoreh Dei’ah 376, Zohar, Volume 3, 115b, Tractate Semachot, Chapter 6) The mourner is lamenting the loss of the opportunity to perform the Mitzvah for a live person.

The mourner is stating that with the absence of the deceased the Presence of God has been diminished; the deceased brought a unique aspect of God’s light to the world. The Kaddish is a prayer for the restoration of all the light the deceased added to the world. (S.Y. Agnon, The Days of Awe)

The act of Sanctification, the declaration of God’s Justice, and the prayer for God’s name to expand in creation all are credited to the deceased, and serves to soften any harsh judgments. (Sifrei; Deuteronomy 21:8, TBKetubot 103a, Tanchuma; Ha’azinu 1).