Concept Art

While many films, particularly those with fanciful settings, are visually spectacular, sometimes, when concept art is released, I personally find that the rejected concepts are much more spectacular than the final product. What are some deciding factors for what makes it to the final film and what remains an illustration?

The work of Concept Artists is one of passion, hard-work, determination and patience. They come up with incredible designs that really help the production and give a visual stimuli to directors, producers, set designers, production managers and art directors. They manage to translate the director's and screenwriter's vision into beautiful images that are then used as a basis to convey the film's atmosphere and tone. They also help the technicians (lighting, camera work) and touch upon all aspects of production. It would be interesting if someone decided to shed a bit more light on these works. What is a concept artist? What do they achieve? How do they impact? Also, would be really interesting to find the concept art for big films to illustrate the point, like Harry Potter, Lord of The Rings or Marvel Universe films etc…