Ordinarily, your camera is programmed to fire the flash and then leave the shutter open for about 1/60 second. Sometimes, though, using the default results in an oasis of light in the foreground, set against a completely dark background, as if your subject were standing at the very entrance of Hades. A longer shutter speed can really hit the spot. Consider a nighttime or low-light scene in which you're trying to illuminate both a person in the foreground and the more distant background. As you know, the flash has a limited range, so it won't light the background. Here's an example of a typical shot in the early evening with the flash firing in automatic mode.