The main reason for increasing the ISO in dark conditions is to make sure you can shoot at a suitable shutter speed. At the same time, you’ll want to shoot at the lowest ISO you can, to get the best possible picture quality. This means you can end up doing complicated mental maths when you should be concentrating on your pictures.
But there’s no need, because your Nikon can do it for you! Auto ISO lets you select the minimum shutter speed you want the camera to use and the maximum ISO, then works out the best combination instantly as you shoot.