I don't agree with forced labor, although there might be incentives put in place for prisoners to do volunteer labor or community service, as others have suggested.
On the other hand, there are some crimes which are so heinous and dastardly that punishment entailing hard labor might appeal to one's sense of justice. One could also argue that, the harsher the punishment, the greater the deterrent to crime - which could be considered a societal benefit.
Occasionally, I see prisoners working along the side of the road, doing clean up and so forth. I've also seen prisoners doing work cleaning at various public buildings downtown.
I actually think there are other ways of going about this. For one thing, there's no reason any non-violent offenders should even be in prison. If they're not violent or dangerous, then they can be put on probation, kept under surveillance, tracked with ankle monitors - yet still be able to have jobs and be more productive than they would be if they were just sitting in prison with nothing to do.