They're originally French I think, the idea is that the water creates an airtight seal which keeps the butter soft and spreadable, but also stops it going off, so you can keep it safely for a month or so - rather than it being kept n the fridge and too hard to spread, or in a normal butter dish and melting/ going rancid in hot weather within a few days (not that we seem to have hot weather this year, but anyway...). I've wanted one for a while but we rarely have butter so it's probably not worth it!
