Auto white balance sounds like it solves every issue regarding colors in your pictures, but it doesn't. For example, when you shoot at sunset or sunrise, AWB wants to 'correct' the golden tones that we love so much when the sun is close to the horizon. It desaturates the yellow and red portion of the spectrum, and the colors look weak and disappointing. By contrast, if you shoot with daylight white balance, you will capture the colors you see. The yellows, reds, and oranges will be saturated and dramatic.