In low light and darkness, our eyes pupil expands to a larger aperture to allow as much light as possible to enter the eye. The dark eye adaption process can take up to 20 minutes for your eye to become accustomed to the low light levels. While this is good for viewing the sky as many more stars will be visible, it simply does not allow you to read any text or star maps. A conventional torch will ruin your "night vision" in a fraction of a second as the intensity of the light will cause the pupil to contract back to normal. It will take another 20 minutes to become accustomed to the low light again.
This is a simple tool that makes a big difference at night. This is nothing more than a LED torch, but it has dual functionality as it emits red or white light. Red light at night will preserve night vision because it offers enough light to see by, but not at an intensity to cause your pupil to contract. This is also why red light is used in dark rooms, as the intensity is not great enough to spoil the film. Many star maps are specially designed to be viewed under red light. The white light function will illuminate a further distance and brighter, useful for clearing up when night vision is not required anymore.