I can touch type in the dark just fine, but many times, staring too long at the laptop screen in the dark causes a lot of eye strain and gives me a headache, even with the screen brightness turned down. In those cases, I often prop up a flashlight to illuminate the area around/behind the screen to reduce the contrast between the screen and dark background if I can't raise the general room illumination. That helps.
In those situations, I have wanted a USB keyboard light for the convenience factor since they are easier to aim than propping up a flashlight somewhere, although your laptop battery life could play a factor if you're not plugged in. I don't like a headlamp for this purpose because of glare.
In addition, if you have to deal with any documents, you need some additional light to read them by.
Of course, for any hunt-and-peck typists out there, it's pretty darn hard to type without light to see the keyboard.