A little while ago Eli Y asked two questions. I was waiting for one of the more learned members of this thread to answer but I don’t think that they were addressed. They were good questions that deserve an explanation. My reply reflects my Bais Yaakov background, there are probably other responses as well.
As a follow-up: What benefit accrues to the deceased by gaining more merit?
Also, do folks in your shul say Kaddish for precisely 11 months?
The benefit to the deceased is this: The honor of Hashem is increased with every kaddish, as explained. Because the deceased is the catalyst for this increased cognizance and appreciation of Hashem in the physical world of action, (where they can no longer achieve any accomplishments on their own,) he/she, in the world of truth, will delight in this added bit of awareness that resulted from their life. I used the term “bit of awareness”, but in the World of Truth it’s not considered small at all. If, the deceased is still in the throes of punishment up there (generally understood mainly as excruciating mortification), because the kaddish has now added additional meaning to the life that he/she lived, this will elevate the soul and thus alleviate some of the pain. The same goes for Yizkor and anything else good that someone does leiloi nishmas someone who has passed away.
There is also the concept that every Jewish soul on this earth, by virtue of being a “son” of Hashem (“Bni bechori Yisroel”), increases the honor of Hashem in this world. So, a loss of a Jewish soul, is also a loss to the honor of G-d. The recital of kaddish helps to restore the equilibrium down here as well.
To my knowledge, kaddish is recited for 11 months following the loss of a parent in all congregations.