Art direction is not only appealing to the player to see when they check out the game, it is also a nice change from the usual "realistic" graphics. 

Legend of Zelda: The Wind Waker shocked and awed players when they saw the game take a cel-shaded approach rather than a realistic approach that teased us all when the game was first rumored. Some fans were offended by the choice of cel-shading but once they experienced the game most were blown away because it seemed to fit so well in the game. You can see the wind wisp by as you sail the open sea watching the ocean waves curl around your boat, it was simply amazing, not to mention an amazing game.

Okami was another that had an excellent and new art style. The game seemed like it was straight from an ancient water coloring book. Vibrant colors surround the wolf protagonist as he runs by lush green meadows, soft pink flowers and as you draw with your paintbrush colors burst out at you initiating attacks, create gusts of wind, draw bombs and other various tasks. It’s truly a beautiful game and definitely brought something new to the gaming world.

Don't get me wrong, realistic games are also nice. How far the gaming world has come in graphics from the 8-bit Mario is astounding and should be displayed. In Legend of Zelda: Twilight Princess the game went for the realistic approach but still had some effects from Wind Waker; how the smoke plumed and a hint of cel-shading was displayed when Link was in the dark world as a wolf.

Art direction is important in a game to keep things new just as innovation. I'd like to see more games that switch different art styles during gameplay to keep new levels/worlds new and interesting.