Very handy if you want to find the Lux when you only have footcandles, and candela or lumens when you got the candle-power. And I thought you can convert candela per square meter (cd/m²) into lux values but you can't. Candelas per square meter involve the reflectivity of a surface, lux is "illumination" which has nothing to do with a surface.

http://www.ambrsoft.com/UnitsConverterCalc/Light.htm