The Pew Charitable Trust does a lot of good research about prison and reentry. In 2011, they published a study that was shocking to those who don't spend a lot of time in prisons. The finding said that visiting an inmate, even once or a few times, dramatically reduces their tendency for antisocial behavior. And, it also said that visits by outsiders reduces the rate in which an inmate is likely to reoffend...also called recidivism. Here's a link to that study.
As a volunteer, it's plain why visiting inmates works wonders....why it it as powerful as education and treatment! It's because prison is a dehumanizing experience, separating inmates from society and (except for staff officers) from members of the opposite sex.
Volunteers, by the very nature of their willingness to come into prison, show that they are open to the idea that inmates are human, that they are seen as redeemable. Inmates quickly understand that outside visitors are ready to accept them on mutual terms of respect. That's not available in very many prisons.
So it is distressing when prison administrators make it difficult on visitors. Yes, the department is responsible for the safety of volunteers as well as the protection of inmates from abuse from outside. But what can happen, unless the administration is progressive, is that policies dictating the conditions of visits by volunteers can discourage or even prevent visitation.
Likewise, when inmates are returned to our communities, it takes many volunteers to help them integrate back into society. It takes willing managers of rental housing, willing employers and understanding social service providers to make the transition to "real" life more attainable. Volunteers, whether visiting in prison or helping in reentry, are crucial in reducing the overpopulation in our prisons. They are, in essence, the front line of prevention, simply by your willingness to make an acquaintance and build some trust.