the real question... why does (sic) pain and agony exist
They exist because we are biological organisms with nervous systems that evolved to detect pain, in order to allow us to detect threats to our survival so as to escape from them in the present and learn to avoid them in the future.
An even more profound existential question however was put forth in 1956 by Frankie Lymon and the Teenagers, but even that is rather easily answered by evolutionary biology.
I think of the future as branching probability streams. - Elon Musk