Those of us left behind can soften the judgment of the departed as well as help our loved-ones rise higher in the supernal realms.
During the first year after death, the primary way for a mourner to do this is through the jewish prayer called Kaddish.
Once your loved-one leaves this world, (Olam Hazeh) the world of action, they can no longer take steps to raise their own spiritual status in Olam Habah, (the World to Come). The mourners left behind are the departed’s only resource to do good deeds (mitzvoth) in their merit.
You will notice in the translation of the Kaddish prayer, there is no mention of death or the deceased. The text of the prayer glorifies and praises G-d. Then what is the connection between this prayer and your loved-one? Since the glorification of G-d is a mitzvoth (good deed), and the mourner would not be saying these praises if not for the deceased, all the merit and reward of this mitzvoth goes towards them.