So writing my own story and thinking of making one classic prison break arc eventually. You know the standard one where extremely powerful criminals managed to break free from their prisons and start rampaging everywhere, and it's up to the hero band to stop them. It's a common event that happens in shonen and fantasy as a whole, giving the protags something to do while exploring the world yada yada.

But eventually one would wonder: why? Why bother to keep such powerful villains captured in the first place. Like most of the time they aren't even the redeemable / everyone should have a second chance type, but like full blown psychotic murder machines with and what not. It's like building your house on top of an active volcano and wait for it to erupt one day. Worst of all, if you actually bother to keep these dangerous criminals around maybe actually have someone equally powerful to keep them in check. But nah, they usually just use common guards. You know those poor sobs that the villain can solo an army of them.

Sure if I cook enough I can think of some reasons to justify it. Such as they are being valuable for certain experiments, or they have too much power that can be extracted into resources, or they hold certain political values or secrets etc etc

The thing is that a lot of series don't even bother to give a reason for it. I suppose it basically becomes a common story beat to the point not many people even question it anyway. "Oh the protags need something to do / someone to fight, here have some op criminals". The funnier thing is that most of the time the protags don't even bother to capture them but also just kill them while fighting, which makes the question back to square one: why bother?