If they would otherwise live healthy lives, why perform the procedure?

Most likely the patient is informed by the practitioner about the risks and they can make an informed decision. That is not the case about ChatGPT where openAI will sell it as the next best thing since sliced bread, with a puny little warning below the page. Even worse when you see all the "AI therapy" apps that pop up everywhere, where the user may think that the AI is as-good as a real therapist, but without any of the responsibility for the company in case of issues.

Because sometimes you don't know with certainty on an individual basis, only a population basis, who is better off from the procedure.

Maybe you are the one in-a-billion who dies from a vaccine, from a disease that unknowably you would never contract or would only contract it in a mild way. The doctors know if they administer it enough times they will absolutely kill someone, but they do it to save the others, although they will pretty much never bluntly put it like that for your consideration.