First and foremost, it is imperative that we understand the essence of a Jewish soul. Specially, one needs to internalize that the Jewish soul does not die when our body expires. Rather, it remains intact and longs for elevation in the next world. As such, the Jewish soul, separated from our body, seeks to constantly ascend and reach closer to our Creator, G_D.

Each time the Kaddish prayer is recited properly, the soul of our loved one becomes exalted, and goes up a level. With mourners reciting the prayer, coupled with others answering amen, this creates an affirmation and testimony in our creator. This affirmation, in the merit of the loved ones, fuels the soul's ascent of our loved ones, each time the Kaddish is recited.

(There is a is a very powerful midrash Tanchuma on the power of Kaddish in Parshas Noach)

Many people believe that when their loved ones die, there is nothing further to do for them. However, the opposite is true. Our Torah teaches us that the mourners have the power to elevate the soul of their deceased loved one by reciting the Mourners Kaddish. Surely, we would want to help our deceased loved ones, in any way that we could.

When we recite Kaddish with proper concentration, we are in effect, ascribing to G-d's greatness, and marshalling the powerful energy toward elevating the soul of our loved ones.

We are in effect saying that, though I have lost my loved one, I still ascribe G-d as the just ruler of the world, and His Name should grow greater and greater. This act of commitment to G-d is a tremendous source of merit to your loved one and is an elevation for their soul. When Kaddish is said on behalf of your loved one, you are doing an action which says "you care", in a tangible way. While your loved one is deceased, you are still able to perform this true act of kindness.

Some say that if one thinks while or before saying Kaddish that he is helping the deceased, he loses everything. Why? Because his intent should be just for sake of heaven. (Gesher ha'Chayim 30, footnote 2 and B'Tzel ha'Chachmah 5:25 citing Yad Eliyahu.) and he should feel pain over Galus ha'Shechinah (Aruch ha'Shulchan 55:4) and yearn for sancitification of G_ds name. Others encourage intending to honor one's parent through saying Kaddish. (B'Tzel ha'Chachmah 5:25, based on Shulchano ha'Tahor and Salmas Chaim, permits for a parent, for the Torah commands one to honor his or her parents. It seems that the same applies to others that one must honor, e.g. a grandparent, parent in law or older brother, but not to other relatives, or a stranger for whom one was hired to say Kaddish.)

Below, are just a few examples in our holy Torah that elucidate the power and importance of a mourner reciting Kaddish:

Our Talmud, Masekta Brachos, teaches us that by reciting the words in Kaddish "Amen, Yhei Shmei Rabba" ("May His great Name be blessed forever and ever") with deep sincerity, this has the ability to combat a lifetime of hardships, and unlock the gates of mercy (mercy or paradise).

Berachos 57a says that one who answers, "Amen, Yehei Shemei Raba Mevorach", is guaranteed a place in Olam ha'Ba. Shabbos 119b says that if one answers it with all his Ko'ach (strength/Kavanah), his decree is torn up.

The Zohar (Terumah 129:2) teaches that by reciting in Kaddish the words "Yhei Shmei Rabbah", that this can eradicate evil forces, which surfaced because of human errors (ethical shortcomings).