There are many cases in which conventional medicine works just fine. The doctor says take these, do this, don't do that, and you'll feel better.
But it doesn't work for all of the people all of the time, and that's what sends people in search of less-conventional treatments. Freelance journalist Melanie Warner explored the cases of people who felt better despite evidence that little had changed for them physically.
Her book is The Magic Feather Effect: The Science of Alternative Medicine and the Surprising Power of Belief. Think placebo effect and a whole lot more; we revisit the author's interview from 2019.