The stated function of a prison is to imprison (i.e. detain).

There are several functions commonly ascribed to prisons, including:

• Detention: to prevent people with criminal tendencies from having the opportunity to commit crimes against the general public, by physically separating them from the public.
• Deterrence: to deprive criminals of the pleasures of normal society, in order to discourage other people from becoming criminals. If you would like to live with your partner, children, and friends in relative comfort instead of with a cellmate in relative discomfort, you have a motivation for staying out of prison.
• Rehabilitation: to cure criminals of tendencies that may lead them to commit crimes; for instance, lack of cultural or moral education, or lack of non-criminal job skills. This is given as a reason for prisons to offer classes, job training, etc.
• Penitence: to put criminals in an isolating environment where they will reflect on their crimes and regret them — or a panoptic environment in which they will internalize the conduct standards of the authorities.

(I'm not disagreeing with you on the badness of prison brutality; just on the "stated function" claim.)