Tonight, which is the first night after the end of the period of saying Kaddish for my late mother, Gwen Montagu נעמי בת אברהם ושרה נ"ע, I was meditating and had the following insight. I hope I will be able to write it down as clearly as it appeared to me.
כִּי עִמְּךָ מְקוֹר חַיִּים בְּאוֹרְךָ נִרְאֶה אוֹר (תהלים ל"ו י')
For with You is the source of life; by Your light we see light. (Psalm 36, 10)
Most people today other than the ultra-Orthodox think of mourning practices in general, and Kaddish in particular, as being for the benefit of the living, as a process in which the bereaved reaffirm their belief in God and God's justice over the course of eleven months, to aid themselves in gradually coming to terms with their loss.
Classical Jewish sources give a completely different perspective: of Kaddish as for the benefit of the dead, as something which alleviates the judgment of the wicked and helps the souls of the righteous ascend from level to level in the Garden of Eden.
My insight tonight was that there is no contradiction. If the bereaved relatives are full of grief and unable to move on in their lives, their parent's soul will be unable to move on in the next world, because it will be too concerned for them and will be searching for ways to pass down comfort to them. By finding comfort through the mourning process, the children are relieving the parent's soul of that responsibility and allowing it to move on to its own next destination.
I don't know quite what to make of the fact that at much the same time I was having these thoughts, a dear friend was posting to me a pointer to the last paragraph of this article, which says almost exactly the same thing from a Catholic perspective.