When a baby is in the mother's womb, it familiarizes itself with her smell. After it is born, the smell of the mother's milk shows the baby where to go to suck it. Throughout its life, a child bonds through smell with its mother and the rest of its family - which perhaps makes it less surprising that Keith Richards, who had lost his father and had decided to plant the ashes with a tree, had snorted the little bits that had fallen on to the table as he opened the box.
Smell plays a very unconscious role in the attachment of a child to its family. Even after a child has left home, it's still not difficult for him or her to recall what the parents smelled like, or the house itself. Odor molecules cause a chemical reaction when they interact with our sensory glands, something that triggers a reaction in the memory of a person making it far harder to forget than sound or sight. Not only that but humans are naturally attracted and responsive to the pheremones and chemicals such as oxytocin, vasopressin, prolactin, opioids, and other chemicals to form lasting bonds that are cemented by touch.
It would be logical to assume that with the final use of his sense of smell, along with a "See you, Dad," Keith had finally reached a resolution with the death of his father - he had been previously unsure what to do with the ashes. Through the final chemical contact, he said his good byes, the attachment not quite severed but not as painful as it is after the death of a close loved one, the cycle coming to an end.