The need of flash arises in the cases when one clicks picture indoors. There are two main sources of light – external flash and in-built flash. The external flash bounces the light from wall, ceiling, floor and similar objects to give an excellent lighting effect to the photograph. The in-built flash or on camera flash are the best alternative to get the proper exposure in case of dim light. External flash however gives less red eye-effect than the in-built flashes. The digital cameras have this feature to set the lighting automatically and settings are also asked to be done for indoors, outdoors and night mode. Flash thus is used as a primary light source in case if light is negligible. In case if shoot is to be taken against the sun then it balances the light automatically. By understanding the mechanics of the flash one can leave the dynamic impression on the image. Thus flash is important to give a lighting effect and light is important to give a natural effect to the picture. In this way both of them, the light and the flash have equal importance in the field of photography.
It depends. I used to rely on flash an awful lot for shooting things, especially people. I’ve kind of grown out of the habit now and tend to rely more on natural lighting and lighting I’ve brought with me now. I do still use it occasionally though.
Like others have noted, it really depends on the situation. For example if I’m trying to take a picture of the night sky, I doubt the flash would serve any purpose, unless I plan to bounce it off the moon.