Why are there prisons? An expert explains the history of using 'correctional' facilities

They thought that society would be safer if more people were sent to prison...

"They" are right. Many murderers and rapists don't stop at one victim. People ARE safer with these people behind bars.

People started spending very long periods of time in prison,

Because many states starting abolishing the death penalty and those that retained it did fewer proportionate executions.

And that's my "supplementary explainer"!

A lot can be said on this topic.

Prisons can be used to punish those deemed suitable for punishment by the powers that be.

Prisons can be used to protect the public from those who have been shown to use the public to prey upon.

Ideally, prisons can be used to "rehabilitate," however one wants to define rehabilitation. Scandinavian prisons seem to have had some success at this, but their societies are far, far different from what most people in the West are used.

And, prisons can also be used to make some people rich. The privatization of incarceration facilities incentivized the locking up of individuals as a way of making money for other individuals. Not the best thing to come out of the Reagan presidency.

The Corrections Corporation of America (CCA) was the first private corrections company, started in 1983. Today it is the largest private correctional company, generating almost 2 billion dollars of profit in 1921. After finding them abusive and ineffective, California banned private prisons in the state, but that ban was held to be unconstitutional by the court. So, the practice of jailing people in order to make money continues.