Determinism is challenged at its most dangerous point: the claim that a person is fully explained, fully caused, and therefore not responsible.
Conditions shape you, but they do not completely write you. Even a narrow residue of freedom can matter, because it changes what you choose to become next.
The argument is practical: if you treat people as fully determined, you train them to abdicate responsibility.
Freedom without responsibility becomes arbitrariness. Responsibility without freedom becomes cruelty. Human life sits in the tension between them.