Light from any source – a window, a continuously burning lamp, a flash – foltows the same general rule: The light falls off (gets dimmer) the farther the light source is from an object. But light from a flash comes and goes so fast that you can’t see the effect of the flash on a scene at the time you are taking the picture. Special exposure meters are designed for use with flash; you can’t use an ordinary exposure meter to meas.
Artificial light sources let you bring your own light with you when the sun goes down, when you photograph in a relatively dark room, or when you need just a little more light than is available naturally. Different sources produce light of different colour balances, an important factor if you are using colour films.
Flash can be used in conjunction with ambient (already existing) light. It can be very useful as a secondary light source to fill in the shadows of natural light, without losing the basic light & shade patterns nor the general atmosphere.