If the only purpose of having a portrait done is to create a good likeness, then a well-taken photograph with good angles and lighting would do the trick. The differences between a professional photo (or a good snapshot) and a drawn portrait are many. When I start a portrait, many times I have never seen the subject in person (or animal, whichever the case may be). I may have only a set of photos to work with, often of average quality. At that point, I rely on my experience as an artist who is familiar with the underlying principles of drawing a face, be it animal or human. First, I must take a 2-dimensional image and make it look 3-dimensional. This applies to anything, living or inanimate. This takes precedence. Keeping this in mind, I study the light, shadow, shades of color, and surface detail. I also have knowledge of the anatomy of a face, superficially and of the underlying structure that gives it form. I am careful not to over commit to any aspect of the drawing, either color or black and white. Details must wait until near the finish. Unlike a photograph, I can choose which details are unnecessary or distracting, and minimize or even omit them from the composition.
That being said, I know I have succeeded as a portrait artist when I turn over the finished piece to the client, they take a look, and the reaction is either a hug or tears! This is common! Then I know I have succeeded in bringing out everything that makes their special person or pet, well, special.