Nowadays, photo manipulation is done perhaps more liberally than ever before due to the spoils of the digital age. When done with restriction these manipulations can indeed help the quality of a photographic image by overcoming some of the inherited flaws of the medium itself. When done out of proportion it usually destroys an image by irresponsibly destroying the character of what was once in front of the camera. Yet, when image manipulation is overly done by overly talented artists then the photographic image mutates and becomes an artifact of the artist's imagination in a collage of resources at his or her disposal.
In so many words and in general, a photograph with little image manipulation tends to be more about what was in front of the camera as the camera recorded it at the precise moment of the capture. Conversely, a photograph with much image manipulation tends to place more importance on the concepts inside the photographer's head...it is more about what's behind the camera.