What I do think is a HUGE amount of cures and treatments are 50-100% placebo, and that most of the money and time spent on healthcare is for things which the body would have fixed itself with enough time, and proper exercise, nutrition, sleep and rest. I think a huge percent of healthcare is really just about making people feel better, whether it is treating symptoms or even just making them feel emotionally better because someone is "doing" something....
The essay I alluded to earlier, to further support my claim that 90% of (1st world) health care is unnecessary:
http://biodieselhauling.blogspot.com/2014/04/healthcare-last-mainstream-superstition.html
I think part of what makes this tricky is: What do you mean by health care, and what you mean by necessary?
Most outpatient visits are for minor conditions, and most people want reassurance or treatment for symptoms.
Is it unnecessary health care if a worried patient comes in with a new rash? How about a cough with fever? How about some mild wheezing? How about a big nontender lymph node in the neck? Any of those could be benign watch and wait things, or they could be signs of cutaneous lymphoma, bacterial pneumonia, asthma, Hodgkin's lymphoma, etc. Any of those could kill someone if not identified and treated. If the doc says watch and wait, then was the care necessary? If they work it up and it turns out to be nothing, was it necessary? If it turns out to be one of the more serious illnesses above, then was the care necessary?
What if someone comes to the doctor wanting advice about wieght loss? Comes in to have their infant's growth and development monitored and receive immunizations? Has cholesterol screening?
I think in the end it gets pretty arbitrary. There is science, there is art, there is psychology, and what is "necessary" varies greatly based on the patient.