What Good does Prison do?

What. So we should just let murderers roam the streets?

the goal of prison is not punishment, but rehabilitation. There is a remnant of this goal in the name corrections facility. But punishment is easy, rehabilitation is hard so we focus a bit more on the other goal, the one saved for those who cannot be rehabilitated back into society: isolation and removal.

If the goal is rehab, why are their sentences based on time, not progress?