Could anyone give a little insight to the thinking here...

I am trying to improve my skills as a photographer/videographer and I have started to film and photograph some indoor glamour/swimsuit type stuff and I am experimenting with different lighting.

Usually indoors I hear people preaching that generally as much light as possible is good so that (with video) the footage is not noisy and dark etc, and to make use of windows and natural light. However, I often see with (sorry to go here) porn, whilst making my (ahem) research that often the action is filmed in front of a window, but rather than pointing the camera at the well-lit side of the subject, the shot is filmed with the window behind the subject. What is the thinking here? Is this so that the shaded side has a softer, more flattering light? Even though much darker?

Can anyone give a little insight as to why the brighter side of the subject is not the one filmed?