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.
I've been leaving butter on the shelf for many, many years. And my mom before me and her mom before her. No issues.
We don't leave it uncovered.
The longest a stick has sat in my house is probably a month. No noticeable issues.
Maybe 2 months is where it turns rancid, I've never got there. But my point is that a blanket statement that butter left at room temp is unsafe is I am quite sure incorrect. Certainly it can't be left out indefinitely, but it can be left out for weeks at a time with no ill effect.