For visitors from UK to the US, they allow UK (220-240V) equipment to be powered from US (and other) 110/120V mains outlets.
These also include: Bahamas, Barbados, Bermuda, Belize, Canada, Columbia, Dominican Republic, Indonesia, Jamaica, Japan, South Korea, Liberia, Libya, Mexico, Nicaragua, Panama, Philippines, Taiwan, Venezuela.
45W Maximum Power
Power supply: 110Vac
Output voltage: 220Vac - 45w
Dimensions: 104 x 65 x 50mm
Electric shavers, small radios, etc - please always check the actual rating!