-Most prison sentences are not life sentences. What one experiences in prison, what thoughts they've had, what choices they made and actions they've taken will affect who and what they are when they are released.
-In prison, one is often not confined to an "8x9 cell", and rather in the presence of other human beings while experiencing other areas of the prison (dayroom/ yard/ kitchen/ showers/ halls and stairways, etc)
-Even when one is confined to their cell (whether solitary or gen pop) there is tremendous opportunity for introspection, contemplation, reflection, formulation, brainstorming, etc...
-Despite whatever "freedoms" might appear to be lost, there are an infinite number of choices that can be made in prison. How one chooses to use one's time, how one chooses to think and behave, how one chooses to interact with the environment, how one chooses to interact with prison staff or other inmates, how one chooses to make use of their connections on the outside, etc.
-Some people feel more "free" in prison than they do on the outside.
-The constant threat of danger, injury and death within many prisons can make an individual far more stronger and capable, as one learns to embrace one's strengths and cover one's weaknesses, as one does what one must do to survive. This process facilitates evolution in nature, exponentially.
-Prison often presents opportunities for field experience, personal growth, character development, knowledge acquisition, contacts and connections, and career oppurtunities for many people ;-)