A Built in Flash is included with every digital camera. The cameras computer determines the need for Flash according to the exposure metering, focusing and zoom system.
On compact camera, the Built in Flash is triggered to go off in perfect sync with the shutter, but it's hard to control the timing and intensity of the Flash. This can result in washes out photos.
The DSLRS have a pop up flashes that can be controlled in various ways to be sync with the drag behind the shutter
The external Flash
Using the Flash helps to illustrate the subject, who can be otherwise appear darker than the brigher surrounding or in shadow. Use the Flash to create shadows and highlights. You can play around with the flash to create cool effects in photos.
