You put the butter in the bell part. The butter sticks. You then put some water in the crock and put the bell in the crock. The bell/butter displaces some water and it prevents air from getting to the butter.
Imo it's silly; there's no real issue just leaving butter in a dish at room temp.
Not really. They were replying to it being unsafe. Sure, they did mention in the comment before that about leaving it in a dish, but a dish that covers butter without the water works too, just not as well as the water one, but better than out in the open. In my experience even out in the open isn't bad in a few days like it says, closer to 1-2 weeks, especially if salted.