The power of the Kaddish Yatom for the deceased is very great!
The power of the great Mitzvah of saying Kaddish to protect and save the soul of the deceased, is very great. However, saying the Kaddish should not be only for the benefit of the soul of the deceased, the main part of the Kaddish is praise and prayers that the name of God should be increased. In the words: “יהא שלמא רבא וכו’ ועל כל ישראל” can be aimed for the soul of the deceased, as well as in the words: “עושה שלום במרומיו”.

The main benefit of a Kaddish Yatom.
Another thing in saying the Kaddish is צידוק הדין, as it’s written in ‘Sefer Hachaim’ (the end of chapter 8) and these are his words: “עיקר תועלת הקדיש לנפש הנפטר היא לפי שאין מיתה בלא חטא. רוב ההולכים לעולמם נפטרים בגלל עוון חילול השם, שאין לו כפרה, אלא מיתה. וכאשר הקב”ה נפרע מהחוטא, שמו של הקב”ה מתקדש, שנאמר: “ונשפטתי”, ואח”כ: “והתגדלתי והתקדשתי”.

That is why the son of the deceased stands and says: “יתגדל ויתקדש שמיה רבא”, that is: I console myself for the death of my father, since my father’s name in heaven is enlarged and sanctified by his death, and the sin Chilul Hashem is immediately atoned for, and so it was during the time of the Sanhedrin, after the judges decided a death punishment, the members of the condemned’s family should come to the Beth Din and say: “You have judged well”, to show that they have nothing in their hearts on the judges, and this is also what ends “בחייכון וביומיכון” וכו’, that is, you Beit Israel, be careful in your actions that you do not cause death to your souls, Just live in the fear of God.”

The main point of the Kaddish Yatom is “Kiddush Hashem”
The regulation of saying the Kaddish is based solely on the idea of making Kiddush Hashem plural. The son of the deceased consecrates the name of Hashem by inviting the entire audience to bless God, with the commandment “ברכו” (which is said after the Kaddish in the morning and evening prayers), or to affirm all the praises in the Kaddish by saying “אמן”. The Kiddush Hashem in public will prove that the tree that produced such fruit (that is, the parents who gave birth to such a son) was not completely worthless.

This fact may have a positive effect on the judgments that God, the judge of all the earth, will make upon their actions.

This is how it affects the soul of the deceased
According to tradition, the soul of the deceased reaches its proper purpose and rest, under the wings of the Shechinah, only after the times of purification. The ascension from this world, the world of vanity and lies, to the world to come, the world of truth, takes the soul of the deceased through the valley of woe, reward and punishment, because “There is no righteous man in the land, who does good and does not sin.” Therefore, the son must always help, and especially in the year of mourning and on Yaharzeit, by virtue of his rights, he can shorten the purification days.

The son should at the path of fear, the son should dedicate time to learn Torah, the son should pray and say Kaddish. Through all these things the son is benifiting the soul.”