I have a question about using mental illness as a defense in court. I understand that mens rea is important in a crime. I understand that spontaneous vs premeditated makes sense as a defense. I understand that if someone is forced to commit a crime, by a 3rd party, that that is a defense as well. What I DON'T understand is the insanity plea. Insanity, in most courts and jurisdictions, means this:
The insanity defense refers to a defense that a defendant can plead in a criminal trial. In an insanity defense, the defendant admits the action but asserts a lack of culpability based on mental illness.
(copied from Cornell)

Why do our courts care if a crime is committed due to mental illness. This is a genuine question. I do not understand why if someone commits a crime, but has mental illness, that the mental illness goes into questioning. A crime is a crime, and I cannot wrap my head around why mentally ill people are prioritized in a legal handicap.