If you have never been inside of a prison, there are things you will not know about the community there. You may guess at them, but that is not the same. What it feels like. What it sounds like. What goes on there; these all define portions of what it is. These definitions, or parameters of life inside, come to you quite viscerally. You feel them in and through your skin before you actually give word or shape to understanding them. You sense before you think.
The themes that come from a prison poet are varied. Most poems you would not have to know the poet was a prisoner to gain access to the import of the word-pictures. Human experience, while diverse, shares some common archetypal qualities. But, some will grow in meaning knowing where the poems were planted. I think themes about being captive are universal, but when you know the poet is in a prison, it can open you to listen differently. Is that a good thing? I don't know. But it is true.