The ISO Sensitivity Auto Control (ISO-AUTO), found under the Shooting Menu, is a powerful feature in many Nikon DSLR cameras. It's used to allow the camera to automatically control the ISO sensitivity and shutter speed, according to the light levels sensed by the camera. It's very helpful when you don't have time to deal with exposure issues—yet must get the pictures.
A good explanation to Nikon Auto ISO setting.
It is useful when shooting with a prime lens (35mm f/1.8) when the light conditions change because I know the minimum shutter speed I need to prevent blur.
However, I disable it when I use a zoom as the minimum shutter speed varies too much depending on the focal length (esp. with my 70-300mm).