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.