Being ill is like waking up in a foreign country, and simply giving that place a name restores a measure of control, whether or not a cure comes with it. Often a diagnosis is all a good doctor has to offer, and it is still worth a great deal.
Careful diagnosis produces a list of possible explanations, and by definition most of them will be wrong. Doctors are regularly wrong in the pursuit of being right, so the discipline is to keep asking: if it isn't that, what else could it be? Uncertainty is not a failure of the method but the water it swims in.
Knowledge is essential but never sufficient, because no single doctor can know everything. That is why, despite all the technology, the tools doctors rely on most are a phone and a respected colleague, mentor or friend who might see what they missed.
The patient's own account of their illness is the most essential evidence, not a pack of lies to be seen through. Getting that story is both absolutely necessary and surprisingly hard, and it is attentive listening, more than machines, that most often cracks the case.
Medicine is a sensual science: doctors gather data through touch and the other senses, following a systematic method. Physical exams compete well with scans and lab tests, so the case is for using both, not abandoning the hands for the machines.