The UNSC flashlight is used in places that are normaly to hard to see. Most of the UNSC guns have a flashlight attached to it, either under or over the barrel.
Halo: Combat Evolved
In Halo: Combat Evolved, the Master Chief needed to get around the Ring mostly in the dark. With his trusty Assault Rifle, the Chief could, with the flip of a switch, turn on a solar powered flashlight. The flashlight was a very useful tool. Considering the amount of darkness in some of the levels, Master Chief needed the flashlight charged at all times. It saved his skin many times.
Halo 2
In Halo, Master Chief's armor was destroyed. Needing a quick repair, he stopped aboard Cairo Station, and got new armor. His new armor was SO much better than his old armor. With his new helmet, he got a flashlight installed in the helmet. This proved a lot more useful, as the flashlight did not need recharging. It also would turn on, and stay on, in mid, to total darkness. It would turn off automatically if to much light would be in the room. It was most useful in High Charity.
Halo 3
The flashlight will no doubt return in Halo 3. If it will be the automatic or solar powered flashlight, is still unknown. It is also unknown what button will be the flashlight.
Multiplayer
In Halo and Halo 2, the flashlight was not a function. This proved both good and bad. The good part was that it would not give your position away. The bad part was that most of the Multiplayer levels were dark, and hard to see enemies. Halo 3 looks to be the same as Halo 2. There will be no flashlight in the Halo 3 mulitplayer. (Assumed)
The Arbiter
Probably the most frustrating thing about the Arbiter levels, is that a lot, if not all of them, were dark. Some levels were so dark, that the player had to rely on dead bodies on the ground, to guide were they haven't been yet. They would also have to rely on running near the wall, to find a door, or corner. But, he did have the temporary cameo shield. That balanced out the missing flashlight function. But, still, it was pretty annoying.