A flash that rotates and swivels lets you bounce flash off walls and ceilings for softer light and more interesting effects. Another way to use external flash is to buy a slave flash unit that fires automatically when it senses the burst of flash from the camera's built-in flash unit. These units are more powerful than the on-camera flash and also allow you to rotate the head to use bounce flash to soften shadows. A slave flash unit. Courtesy of Digi-Slave .
Every flash has a maximum useful range.
How bright the light from a flash is when it reaches a subject depends on the flash's power and on how far the light has to travel. The further the subject is from the flash, the less light will reach it and so the less light will be reflected from the subject back toward the camera. Objects near the flash will be lighter in a picture than objects farther away. When the flash fires, the beam of light expands as it moves father from the camera. As a result, subjects nearer the flash will be illuminated with a more intense light than subjects farther away.