When you’re shooting with flash, it’s really important to consider how much ambient light you want to “let in” to your photograph, and that means adjusting your shutter speed. In these two examples, taken seconds apart, all of my settings are exactly the same with exception of my shutter speed.
The image on the right was shot at 1/250th of a second, while the other image with the lighter background was shot at 1/60th. The flash power and aperture are the same. See the difference? The slower the shutter speed, the more ambient light seeps into the image. In this case, they both work because the sunset behind the model is quite beautiful as well, but each photo has a very distinct feel to it. With this lighting, had I changed my shutter speed to around 1/10th, the sky would have been overexposed and I would have lost the gorgeous sunset in the background!