The most effective and only real deterrent to crime is the presence of authority such as police, guards etc that can catch and arrest the criminal for doing the crime.
The actual punishment for the criminal is not enough deterrent for crime as some criminals do not really care about having to spend some time in prison.
Prison and Jail and even fines are not enough to deter a criminal.
And evidence has shown that the threat of being put to death by execution is not even enough to prevent crimes such as murder etc.
Prisons are not effective in preventing crime.
Although some people fear the possibility of prison or jail and that is enough to deter some people from committing crime and some people who commit crimes for the first time and go to jail may decide it's not worth it.
That can sometimes be enough for some people to avoid getting into trouble.
However some prisoners who are in prison for a long time and then released may actually commit more crime after they are released.
There have been prisoners who have served 20 years or longer in prison for robbing a bank and then they got out and then committed another robbery within a few days or so.
Then they got back in prison for another 20 years or so.
Being around other criminals in prison actually makes it more likely for the prisoner to commit another crime of the same crime or worse when they get out.
We do need prison systems though for really dangerous people that would be bad for society.
But prisons do not rehabilitate people or correct their behavior like the name correctional facility insists.