Auto white balance is great for color. I hear this all the time: "I shoot RAW, so I can shoot auto white balance and not have any problems." While it's true that you can easily change your white balance in any program that processes RAW files, the challenge is that you have to do the processing and you have to have some reference to make sure the processing is accurate or even appropriate. Auto white balance has two main issues when shooting outdoors.
First, it's inconsistent. If you photograph a landscape with flowers in the foreground and shoot with a wide-angle lens, then use a telephoto on just the flowers, you'll discover the flowers have changed color because the camera has changed the white balance. White balance is designed to change, and that's a benefit indoors where things like fluorescent lights don't have consistent color.
Second, auto white balance has a tendency to add a slight blue cast to scenes, especially under cloudy and shady conditions. This blue cast makes neutral colors no longer neutral and damages the saturation of warm colors. A big problem with this blue cast is the way our eyes look at images on the computer screen. Our eyes are very adaptable, and unless there's a standard reference to work against, our eyes adapt to that blue cast and think it looks okay even though it isn't, so we don't adjust properly. Even if you do make the adjustment, you run into the problem of the inconsistency of auto white balance, making it impossible to know which photo is correct.
Shooting a specific white balance, such as "Sun" for sun, "Shade" for shade, "Cloudy" for cloudy and so forth, locks in your white balance to a specific point and ensures you don't have the unwanted blue cast. Cloudy white balance is also great for locking in colors at sunrise and sunset that are closer to what we expect from film.