In my portraits, I almost always use flash to build on the light existing in a scene, to direct the viewer's eye around an image and create a hierarchy of information about the subject. Even outdoors, the addition of lighting can make a difference in the way an image communicates.  In some situations there are crowded backgrounds to consider. That was definitely the case when I  photographed scrap metal workers in their warehouse as it was a jumble of machinery and metal: Engine parts commingled with construction debris and disused farm implements. Here the flash highlights the subject, Joe (and the circuit boards to his right), bringing them out of the chaotic background. The use of lighting to help organize information in the frame is an important part of my process in creating environmental portraits.