Prisons are a nuanced topic that cannot be boiled down to categorical statements, but I'll do my best here to clarify.

You have 2 different cases for prisons: prisons as a concept and prisons in reality.

Conceptually prisons serve 2 important roles: separation and rehabilitation. Both roles are important for the continued functioning of society. You cannot have a functioning society with no separation of criminals from the population. Similarly without rehabilitation the separation needs to be permanent. In some cases there is no rehabilitating someone, so life sentences exist.

In reality: only a very small subset of prisons match the conceptual purpose because there is still a strong group of voters who think a prison should be a punishment.

To consider a specific case, let's take the infamous US prison system. In the United States rehabilitation is the exception, not the norm. Beyond that the carceral system has perverse incentives to perform that role of societal separation on the maximum number of humans possible without concern for innocence. It's not an accountable system and it is not democratic.

Even with those perverse incentives: you still have prisoners in prison who would need to be there even in the most perfect system. Plenty of people in prison need to be there. The system fails to rehabilitate them and only serves to separate them from society, but that role of separation is an important one.

I'd argue that the US prison system is overwhelmingly negative for the society but it still performs a core societal role. Despite that: I personally know excons who have had dramatic changes from time served and are better people for the experience. Some percentage of the population benefits.

I don't buy into anarchist utopian handwaving that states that prisons aren't necessary: people suck and would suck regardless of governmental style.

Does that help clarify?