A device needed by UK blokes (or people from any other ~220V country) when they buy a an imported games console that doesn't use an external transformer (pretty much all of them these days). Turns the 240V mains supply into a 110V supply that the console can use. Also used by early adopters of consumer electronics goods I'd imagine.

A solution to one of the many problems associated with getting stuff the same time as everyone else in the world.