In the beginning of filmmaking, film quality was so bad that it needed a huge amount of light to register details. Only the sun was bright enough. For this reason studios were built in sunny places, such as
California. At that time a studio would have white cloth roofing, to catch the sunlight and diffuse it down onto the stage below. Many early films have very little shadow
definition in them due to the size of the light source, the whole roof.
As film quality improved and powerful artificial light sources became available, studios became less dependent on sunlight, and the lighting cameramen became more able to paint with light. They still used huge amounts of light but were able to control it better.
At this stage the custom of using one keylight developed. This one keylight decided from where shadows were cast throughout the shot. It shaped the shadows on the subjects, and the highlights. Other lights were used as fill, to lighten up shadows and bring forth detail, or to create effects, such as the backlight used to separate the subject more clearly from the background. But the keylight was the ruler of the lights.
Of course the keylight was not always a harsh small light source, it was often softened and enlarged to give a gentler light, especially for shooting such things as a woman’s face. But it was the main light source for the definition of the lighting in the scene.
This custom diminished a little in importance with the development of smaller cameras, better film and more documentary shooting styles, but still holds sway in studio productions and a lot of location shooting.