why would someone pierce their nipples?

This is genuine curiosity.

what does it do besides mutilating an erogenous zone?

The moment you assign a negative conotation, you are passing judgement.

Others have replied saying that having such piercings did not hamper their sensitivity or even increased it.

I mentioned it can even serve to correct inverted nipples.

I respect you do not see such practice in a favorable way but if you don’t like, just don’t do it. Nobody is advocating for it.