I'm not sure where you're from and prison effectiveness tends to be dramatically different throughout the world. However, where I'm from, federal penitentiaries have a first year recidivism rate of over 40% (for non indigenous males) and nearly 60% (for indigenous males). In Canada, the provincial correctional system houses people sentenced to two years less a day, so federal penitentiaries tend to house more violent criminals.

I'll agree that it's nice having them off the streets, though I hate knowing that we release people knowing that on average, over 40% for reoffend within their first year.