In Ernest Hemingway’s "A Clean Well-Lighted Place," why is the "nada" prayer central to understanding the theme of the story and the perspective of the older waiter?

The thematic concerns of Hemingway's 1933 story have to do with the divide between youth and age and whether life has any meaning beyond what we ascribe to it. 
The perspective of the older waiter is that the old man who comes to the cafe to drink deserves to be treated with compassion and dignity. He is sympathetic about the old man's suicide attempt and obvious loneliness and isolation.  The younger waiter is self-absorbed and cares nothing for the old man, seeing him only as a nuisance and an obstacle to getting home to his wife. The older waiter understands that the old man finds meaning in the simple pleasure of enjoying a few brandies in a "clean, well-lighted place," because he has concluded that life is essentially devoid of meaning beyond what we create for ourselves.  The younger waiter is deluded by his own youth; he has yet to figure out that life is meaningless.
The "nada prayer" expresses the older waiter's belief that life is meaningless except for what we choose to make meaningful. He finds purpose in providing a clean and orderly place for the old man and others like him (including himself) to spend time. To him, that is meaningful. The prayer expresses that life is meaningless and that the rituals of religion (praying) and the notion of an afterlife are also meaningless. The tangible phenomena of a drink, light, and a clean place are concrete and meaningful, though temporary.