I think that phrase might be giving the wrong impression. It's not going to be a prison for people. I've got no right to toss people into some prison. This is primarily for situations where we go into a home, find a hostile ghost, or demonic spirit, or any other kind of hostile thing, anchored to an item. Under normal circumstances we'll try to exorcise it right there, but if there are children in the home my first priority will be getting the item out of there so the children are in no danger. The item would go into the storehouse, where we'd exorcise it without risk to the children in the home.
Another example would be if we couldn't handle exorcising the item in question on our own, we'd take the item to our storehouse and call in help from someone better equipped to deal with it. But it is definitely not going to be a prison for people.