The lid sits in water. When you open it the water on the lid gets in the butter.
Also unless you're changing your butter out twice a week and properly washing the bell inbetween, the whole thing will get rancid and gross fast- much faster than butter goes bad in a covered butter dish on the counter.
I just found the whole process with it irritating. Just give me a ceramic butter dish with a lid. Trust me, butter doesn't last long enough here to go bad.