Why do medical doctors need to know anatomy?
So that they can see where problems are.
If they know the anatomy of the body then they will be more able to pinpoint problems and refer the patient on to the correct specialist.
Because you can't learn how to fix something without knowing its structure, same applies to medicine. In order for medical doctors (or other medical professionals as well) to study diseases, they need to understand how the body works and how its structure is. Because of that, anatomy is an essential key factor in understanding how the body works normally and abnormally (diseases).
And almost all medical students hate anatomy.
I hope that answered your question.