But if he just lets it sit, it'll get rusty beyond repair. This brings merit to the deceased by his or her descendant's continuing to do mitzvot in this world. Dear Baruch, The soul, or neshamah , of the departed derives joy from the kindling of lights. This enjoyment brings the neshamah to a state of spiritual expansion. It comes and sees the light burning for it, and receives spiritual satisfaction from this.
Some authorities write that it's best to light the yartzeit candle in the synagogue.
Perhaps this brings merit to the deceased by enhancing the honor and ambiance of the synagogue. My son is beginning to learn Hebrew, and he knows all the letters. But he makes mistakes with the vowels, and with knowing where the words begin and end. There is a midrash that when the king's servants asked Mordechai why he didn't bow to Haman, when even his ancestor Yaakov bowed to Esav, Mordechai replied that he is descended from Binyamin the son of Yaakov who never bowed to anyone in his lifetime. Since my brothers began to bring it home from shul on Friday nights, it has been one of the highlights of my Shabbos.