If the patient is currently taking a medicine that reacts to a certain treatment, it can lead to bad side effects. If you didn't have that medicine in their info, you could unknowingly treat them with something bad.
So the doctor knows what meds to put you on so they don’t negatively interact with the ones you may already be on. And so they’re aware of any past problems you have had and what you’re allergic to etc. it prevents problems from occurring and prevents the patient from being put into harms way.
