History is a basic knowledge that the doctor has to collect from you, to help him diagnose a disorder or assess your risk of developing disease. He may question on your current symptoms, including your past illnesses or treatments you have been taking. If you are visiting your doctor with some symptoms, then your medical history will be of great use to figure out the cause of the symptoms. And if you are visiting for a general check up, you are more likely to be asked questions related to your lifestyle, your health, or even your diet.

Your doctor should know your past health problems, which is related to your current symptoms. Symptoms may be a side effect of a treatment which is going on from past few days. Therefore, it becomes important that you tell your doctor if you are taking any medication or drug related to your past symptoms. You should even tell about the alternative therapies you are taking. He should take care that the new medicine, which he suggests, should not react with any other medicine.

Your doctor may ask family related questions such as illness or cause of death in close relatives leading to any disorders, or hypertension, coronary artery disease and diabetes, etc. If the family history is of a particular condition, he may check that symptoms are not due to that disorder and suggest preventive measures to reduce your future risks of this condition. 

You should tell about your private lifestyle, as it helps the doctor to solve your problem. Questions such as what you eat, what you do, and where do you live etc. can all influence your health. By these conditions, the doctor may help you to come out from your problem.

The doctor may ask about your diet, whether its balanced or not and about exercise. You may also be asked about drinking, smoking and the recreational drugs.

Your symptoms depend on your profession that you do, affecting your health. Even your past occupations may be the cause and effect of your current disease. 
It also depends on your personal relationships or living conditions in your life and even financial conditions, which can also affect your health, mostly your psychological well-being. You should also inform your doctor about your abroad tour if any; your symptoms may be due to any unusual infectious disease.