The Weapon Flashlight can be attached to the Glock 17 pistol, and used as illumination at Night. This can be accomplished by having both the Flaslight and the pistol in your main inventory, and then clicking "Attach" on the Flashlight. This (Should) add the flashlight to the weapon. Please note that this will not work with normal Flashlights.
Although the Flashlight can illuminate large areas, the beam is highly visible from up to a Kilometer away, and can sometimes be viewed from even further distances, (Depending on fog) and it is advised to only use it for short periods, to get your bearings, before turning it off again.