Knowing if a patient has tuberculosis, hepatitis b, or aids is important because?

Short
because prior case should be handled first before anything else, or because a doctor should determine modification of treatment for unusual medical history.

Long
When a patient has a certain history of disease, he/she could have different treatment from other case because he/she might not be suitable for usual treatment. The doctor needs to modify the treatment so it could reach better goal.

The other importance to know medical history is we can avoid what kind of medication patient had before coming to the doctor, so the doctor could avoid medication that interact with the medication patient had before.