A question, Why do we light for photography?

With modern cameras apparently able to "see" in virtually no light (..ahem) a question arose the other day about why we have to add light for photography reasons. I shall now answer this.. 

1) To Illuminate the subject-This sounds obvious, and is, of course, as photography is named after the Greek word photos=light
2) To produce the correct exposure on the film or sensor- see the earlier post about aperture and shutter speed control and making the sensor or film accept the same amount of light
3) To create contrast and depth
4) To create mood

OK, let's start with Illumination
This is obviously the most basic function of lighting. Think of a dark night in a field with no reflected moonlight. It is pitch black and your eyes can see nothing, so you switch on a torch to allow your eyes to register the ground in front of you. Well, a camera sensor works in the same way. The sensor is an electronic panel that has many millions of light sensitive cells on it. The photosensitive cells need to be stimulated by light and in turn, this creates an electrical charge that is converted into a signal. So, in order for a signal to be recorded, there needs to be enough light to make the sensor work correctly. No light=no picture. Simple

Correct exposure
This is controlled through a number of contributing factors including lens aperture, shutter speed, camera filtration, and lighting. You need to introduce enough light onto the subject to allow the camera to perform at its optimum. This means understanding the limitations or 'dynamic range' of a given format or camera.
For example, many cameras can have as much as 8-12 f-stops of dynamic range , that is the difference between resolving detail in both the shadow (dark) area of the scene and the highlight (bright) areas of the scene simultaneously. The human eye can resolve much more than that. It's difficult to measure exactly how much because our eyes adjust so quickly to contrast in a scene, but it is estimated that we can see about 15 f-stops of range in a given scene. The actual number is less relevant than understanding that cameras are much more limited than our eye, and so you have to use lighting to capture the important parts of the scene so that they fall into the 'resolving' capability of the recorded image.

Create contrast
Photography is normally a 2 dimensional medium, yet we experience life in 3 dimensions, with the output of our 2 eyes being continually computed by our brain to experience depth. Setting aside the evolution of 3D shooting and 3D video technology, light is used to create the illusion of a 3rd dimension in 2 dimensional video.
Photographers use light to give the scene a sense of perspective. By creating contrast between shadow and highlight, you give the viewer a sense of depth. A backlight can help "un-stick" a person from the background, hence the need for lighting.

Create Mood
Light colour, light intensity and light quality all contribute to creating mood in a scene. I very much subscribe to the school of thought that believes you should always motivate where light comes from, and use realistic colours that are natural or appear to emulate our environment.