Because it works. Although there are some medications with functions nearly definite and specifically designed (such as imatanib), many of the medications used in the clinics and hospitals of the world are used because they are effective in treating disease and have tolerable side effects not because we fully understand how they work. Aspirin was used for many years in many forms before a good idea of how the medication was likely exerting an effect was worked out. Cancer chemotherapies were originally cocktails of cytotoxic drugs used in varying combinations and concentrations until a more or less effective recipe was obtained (for more on the history of cancer, Emperor of All Maladies is an excellent read). The gold standard then for determining medication is (1) Is the medication safe? (2) Does the medication treat the disease as well or better than the current standard of care? While researchers elucidate the details of how the medication is working, a well understood or worked out mechanism is not required for use in patients. Success through clinical trials requires a demonstration that the medication is safe in small and large groups of patients and demonstrates a therapeutic effect greater than the therapies which are currently available.
In a similar manner we make decisions in our life based on these two questions: (1) is it safe? and (2) is it effective, does it work? We select anything from over the counter medications, to grocery stores, to restaurants, to friends, to cars, to occupations, to schools, to hobbies based on these two questions. Often we don’t require an in depth mechanistic knowledge of how these things work (how good of a car mechanic, pharmacist, or chef are most of us?) The strange thing is, that if you ask most people they will assume that we have an in depth knowledge of these things. This assumption makes it easier to function in a system where the real answer is very likely because it’s safe and because it works.