Stand out or Blend in?
Most photographers want their subjects to stand out from the background, to achieve some depth or interest. This involves a sometimes complex arrangement involving lighting ratios, distance from subject, size of the modifier, and type of modifier. Of the two images above, most would agree that the darker background gives the subject a little more depth and possibly more interest. The subject clearly stands out a little more in the second image compared to the first image. The second image even appear a little warmer than the first image.
The images above were taken with a single light, a medium softbox. Such lighting effects can be accomplished by moving the light a little closer to the subject and allowing light to fall off a bit quicker (inverse square law). A small adjustment (1/2 stop) on the lighting created an different look for this subject. Of course a second softbox or even a reflector could have been added to the rear of the model to soften some of the shadows, with lighting ratios possibly at 1:2 (second light on a lesser power).
But sometimes we may want the subject to blend a little more into the background such as:
It would be difficult to achieve this effect with a single light without significant post processing. This high key image with a pure white background was accomplished with two large umbrellas pointed at the background with an additional large umbrella lighting the subject. Lighting ratio's were close to 1:1 and adjusted to give the subjects skin a very pale look. Very minimal shadows appear even on the skin of the subject. The larger umbrella lighting the subject was several feet away from the subject to achieve a high key effect.
We may want to blend the subject into the background to create yet another look. In this case the model was surrounded by black to absorb as much light as possible, and the single strobe was fitted with a 10% grid to focus light onto the subject. Here much of her body tends to blend in with the darker background.
Depending upon the look and feel of the image, the choice to separate the subject from the background can give a very different feel to the image.