One reason, of course, is to ascertain what facts in the medical history might predict a greater risk of health problems later in life, or be useful to diagnose a current problem, or response to therapy. If our medical history is understood this way, then by extension, our family's medical history is just a way of getting more information about what might happen to us in the future.
But of course the medical history is more than just a series of checkboxes to figure out which conditions can be entered into a predictive risk calculator. It is another way of referring to the entire experience of illness from our (the patient's) perspective.
This goes back to a larger theme that I have discussed here (and in the book) time and time again, the difference between the open-ended and the closed conversation. The closed conversation is meant to collect information that the doctor (or whoever) is sure a priori should be elicited. The open-ended conversation, on the other hand, can turn into something that neither party expected. The doctor might found out that she has more in common with the patient than she thought. And vice versa.
A patient of mine with chronic pain, in her 70s, sees me every month, and to tell the truth I usually focus on the specific markers, both physical and pathological, or her multiple diseases. Until she was hospitalized recently, I never knew that she had served in the military a couple of decades ago, and in that capacity had traveled around the world. She has much to tell, and I never realized it.
Will this cure her pain? No. But I hope with this piece of information, and, ideally, both me and her attuned to the opportunities afforded by it, we will break out of the cycle of test-treat-test-treat frustration which so often hobbles us, especially for chronic conditions where fallacy of a single solution is misleading and often harmful.