Pain is pretty much body-ese for “bad”. It is necessary as an indicator of what is bad—injuries, poisons, illnesses cause pain because pain is how we know those things are bad.
It’s kind of a meaningless question then; the badness is not contained in the pain but in the things causing it.
You might as well ask what is acid about the colour on indicator paper.
Pain is pretty much body-ese for “bad”. It is necessary as an indicator of what is bad—injuries, poisons, illnesses cause pain because pain is how we know those things are bad.
It’s kind of a meaningless question then; the badness is not contained in the pain but in the things causing it.
You might as well ask what is acid about the colour on indicator paper.