It's a diagnosis guideline for a doctor to determine a definitive medical condition.
It's like when a doctor tells you "you have diabetes", except he says "you have depression/anxiety/etc".
Because mental illnesses are much more related to our minds which is our almost-everything, laying a mental diagnosis to a client has a much more unsettling impact.
We need to always remember those are medical diagnoses which are just official paper diagnoses, not mind/soul/consciousness/you/strength-level/intellect-level/etc diagnoses. Just paper-written guidelines for the doctor to treat symptoms according to just protocol guidelines.