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.
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Okay, why does this "alert" not exist as just a signal such as a blinking light on a dashboard of your car?
The need for a signal is just to warn you to tend to some issue that needs fixing or remedying. Theoretically without pain, you would think the quality of life would be of such high value, tending to such an alert would become first priority if it threatened your existence, or the ease of. So from that you could ascertain that pain and agony are not required in the form that we people sense it. In fact, it would if anything, cause slower reaction, and the pain itself would become the quantity of highest concentration, rather than upon the cause of the alert itself. The pain/alert system in the form that we humans sense it, is IMO obsolete an actually contributes moreso to our failures rather than contributing to our survival. That's a little black and white (and could be explored further) but you can probably see what I'm getting at.