I belong to a health care system of providers. Sometimes if I need to be seen quickly, I have to see a different doctor than my primary care physician. Sometimes I also go to urgent care. The result is that the person I am seeing does not really know my medical history, and it is up to me to remember to include all pertinent information. If you are sick, this can be hard to do. You may be in too much pain to think clearly, and leave something out.
I went to a doctor for an ear infection, and I was given ear drops. The drops did not work, and I went back. This time I saw a different doctor. I was given different medicine. When I got home, I realized I did not know if I should use both medicines, or just one. I had been in a significant amount of pain, and didn’t think to ask or didn’t remember. I was given no written instructions other than the prescription bottle and a handout about swimmer’s ear. If I had been given written instructions from my doctor, I would have felt more confident.