If prison sentences are merely a deterrent to other criminals, then sure, we need some evidence to back up the idea that deterrence works. (I would think it's pretty self-evident that there are a ton of minor/medium crimes that would increase in prevalence if there were no consequences, though.)

But it's not. Prison is also a punishment for the person who committed the crime, and, if our prison system wasn't so messed up, could be a place for reform.