People are imprisoned when they are a danger to others or at risk of flight. Their time in prison is meant to reform them and help them back into society. The community is protected from further "mistakes" from a person, while allowing the person to learn not to make the same mistake again.

At least that is the idea.

Some mistakes are accidents, some mistakes are made on purpose.

The latter needs some kind of consequence in place to offset the benefits from purposefully making said mistakes.