One of the four of five essential lenses for pros, broadly useful for artists and accomplished amateurs. Used for landscapes, interiors, street shooting, crowd shots, etc. Also used by bored amateurs as the next thing to covet for purchase. Despite the ubiquity of this focal length, relatively few photographers are practiced enough or visually acute enough to use this type of lens effectively; lots more people own these than do good work with them. See Brian Bowers’ Leica books for a rare example of a scenic photographer who actually sees well with a 21mm.