In traditional film based photography once you’ve loaded the film into the camera you were stuck with all the parameters of that film until the roll was filled with photographs. This meant that if you had a day film (ISO 100 or 200) your night shots required long exposure time or use of flash, which sometimes is not desired at all.
With the emergence of digital photography we can now vary the sensitivity for each frame. This photographs for example of a Asian man in his news stand taken at Sydney Circular Quay was taken at ISO 800. This allowed for quite brisk exposure time and no need for flash. Canon’s sensors (in my case Canon EOS 30D) in particular have a very clean high ISO results and there is no visible noise in the photograph at this setting.