These are are very convenient, simple to use, and can provide a high percentage of successful images. For many forms of wildlife, and in particular birds, fill-flash is frequently needed but difficult to accomplish. Such photography is typically done with long lenses usually between 300mm and 600mm in focal length. This has lead to the recent development of several different devices whose purpose is to concentrate the light output of a flash head so that it has a greater reach. My research turned up what I regard as the best of these, Walt Anderson's Better Beamer available from Art Morris Birds as Art web site.
Art is one of the world's preeminent bird photographers.
This frame shows the nature of the problem. Neither film nor imaging chip can handle the exposure range. If you manually open up by a couple of stops you'll properly expose the monkey, but will burn out the sky. Fill-flash solves the problem. You'll find numerous other photographs there and on linked pages that used the Better Beamer .