In the US? It’s not about prisoner rehabilitation and never has been.

Incarcerated workers are under the complete control of their employers, and they have been stripped of even the most minimal protections against labor exploitation and abuse.

Nationally, incarcerated workers produce more than $2 billion per year in goods and more than $9 billion per year in services for the maintenance of the prisons. https://www.aclu.org/news/human-rights/captive-labor-exploitation-of-incarcerated-workers

I agree that prisoners should be kept active, given opportunities to develop skills, and that setting them up for continued employment after incarceration is a good idea too, but there are better ways than what we’re doing right now.